Rollforward of Fair Value Measurements by Level (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	$ 4,050,900	$ 3,343,600
Total realized and unrealized gains (losses)	(29,600)	(25,900)
Amortization/Accretion	(4,200)	(4,600)
Purchases	(412,400)	447,800
Sales	(1,128,500)	(457,400)
Transfers in	0	5,200
Transfers out	0	(5,200)
Balance at March 31, 2016	3,360,200	3,355,300
Other Investments	321,800		$ 315,800
Net realized and unrealized investment gains, after tax	21,000	17,900
Net realized and unrealized investment gains	29,500	25,000
Short-term investments, at amortized cost (which approximates fair value)	$ 261,700		211,300
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers	(5.00%)
Maximum price difference provided by pricing services	$ 1,000
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	1,152,200	550,600
Total realized and unrealized gains (losses)	(11,300)	10,000
Amortization/Accretion	0	0
Purchases	(109,000)	94,100
Sales	(852,900)	117,700
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2016	419,600	537,000
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	2,531,400	2,372,900
Total realized and unrealized gains (losses)	(19,100)	16,900
Amortization/Accretion	(4,200)	(4,600)
Purchases	(292,400)	317,400
Sales	(272,600)	326,900
Transfers in	0	5,200
Transfers out	0	0
Balance at March 31, 2016	2,566,100	2,380,900
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	70,000	76,400
Total realized and unrealized gains (losses)	(500)	200
Amortization/Accretion	0	0
Purchases	0	29,700
Sales	0	0
Transfers in	0	0
Transfers out	0	(5,200)
Balance at March 31, 2016	70,500	101,100
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	0	39,500
Total realized and unrealized gains (losses)	0	1,800
Amortization/Accretion	0	0
Purchases	0	0
Sales	0	9,300
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2016	0	32,000
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	169,500	110,600
Total realized and unrealized gains (losses)	(1,100)	(6,200)
Amortization/Accretion	0	0
Purchases	(2,100)	1,400
Sales	0	3,000
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2016	172,700	102,800
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	127,800	193,600
Total realized and unrealized gains (losses)	2,400	3,200
Amortization/Accretion	0	0
Purchases	(8,900)	5,200
Sales	(3,000)	500
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2016	131,300	201,500
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	4,134,400
Balance at March 31, 2016	3,490,600
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	1,363,500
Balance at March 31, 2016	681,300
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	2,531,400
Balance at March 31, 2016	2,566,100
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	239,500
Balance at March 31, 2016	243,200
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	2,639,700
Balance at March 31, 2016	2,613,200
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	134,000
Balance at March 31, 2016	72,900
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	2,435,700
Balance at March 31, 2016	2,469,800
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	70,000
Balance at March 31, 2016	70,500
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	169,500
Balance at March 31, 2016	172,700
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	0
Balance at March 31, 2016	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]
Balance at January 1, 2016	0
Balance at March 31, 2016	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]
Balance at January 1, 2016	169,500
Balance at March 31, 2016	172,700
Carrying value of investment accounted for using the equity method	3,700	3,800	3,800	$ 5,200
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	211,300
Balance at March 31, 2016	261,700
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	211,300
Balance at March 31, 2016	261,700
Short-term investments, at amortized cost (which approximates fair value)		453,700	211,300	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]
Balance at January 1, 2016	0
Balance at March 31, 2016	0
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016	0
Balance at March 31, 2016	0
Discontinued Operations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains	(8,800)	78,400
Prospector Offshore Fund Ltd [Member] | Fair value measured on a recurring basis | Investment related liabilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains		800
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	14,100	$ 16,200	14,700	$ 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