Investment Securities (Fair Value Measurement by Level) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 3,360,200	$ 4,050,900	$ 3,355,300	$ 3,343,600
Other Investments	321,800	315,800
Short-term investments, at amortized cost (which approximates fair value)	261,700	211,300
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	419,600	1,152,200	537,000	550,600
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,566,100	2,531,400	2,380,900	2,372,900
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	910,400	1,000,000
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,260,900	1,167,000
Mortgage-backed and asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,260,900	1,167,000
Mortgage-backed and asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70,500	70,000	101,100	76,400
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0	32,000	39,500
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	172,700	169,500	102,800	110,600
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	131,300	127,800
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	131,300	127,800	201,500	193,600
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3,490,600	4,134,400
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	681,300	1,363,500
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,566,100	2,531,400
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	243,200	239,500
Fair value measured on a recurring basis | U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	88,800	160,000
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	72,300	133,400
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	16,500	26,600
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	910,400	1,000,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	910,400	1,000,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	242,100	253,300
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	242,100	253,300
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	135,100	135,600
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	135,100	135,600
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	155,500	175,900
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	155,500	175,900
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	43,300	49,200
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	43,300	49,200
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	124,700	151,300
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	124,700	151,300
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	69,300	82,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	69,300	82,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	57,100	60,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	57,100	60,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	57,200	61,500
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	57,200	61,500
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	26,100	31,200
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	26,100	31,200
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other:
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,260,900	1,167,000
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,260,900	1,167,000
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	268,700	228,800
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	268,700	228,800
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,200	1,200
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	600	600
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	600	600
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	83,200	82,700
Fair value measured on a recurring basis | Preferred Stock | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Preferred Stock | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12,700	12,700
Fair value measured on a recurring basis | Preferred Stock | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70,500	70,000
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,613,200	2,639,700
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	72,900	134,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,469,800	2,435,700
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	70,500	70,000
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	261,700	211,300
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	261,700	211,300
Short-term investments, at amortized cost (which approximates fair value)		211,300	453,700	376,800
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Consumer
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	73,100	70,000
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	73,100	70,000
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	30,000	26,600
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	30,000	26,600
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Financials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12,900	694,700
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12,900	694,700
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Communications [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	34,900	43,700
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	34,900	43,700
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Health Care [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	39,000	35,700
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	39,000	35,700
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	47,800	27,000
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	47,800	27,000
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	130,500	141,800
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	109,000	120,500
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	21,500	21,300
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Other:
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	74,800	74,400
Fair value measured on a recurring basis | Common equity securities | Other: | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Other: | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	74,800	74,400
Fair value measured on a recurring basis | Common equity securities | Other: | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	443,000	1,113,900
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	346,700	1,018,200
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	96,300	95,700
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	172,700	169,500
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	172,700	169,500
Carrying value of investment accounted for using the equity method	3,700	3,800	3,800	5,200
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	14,100	14,700	$ 16,200	$ 16,800
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 14,100	$ 14,700