Rollforward of Fair Value Measurements by Level (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 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)			57,600	11,100
Amortization/Accretion			(8,000)	(9,900)
Purchases			3,622,500	1,293,700
Sales			(2,631,200)	(1,258,900)
Transfers in			0	34,900
Transfers out			0	(34,900)
Balance at June 30, 2016	$ 5,091,800	$ 3,379,600	5,091,800	3,379,600
Other Investments	329,200		329,200		$ 315,800
Net realized and unrealized investment gains (losses), after tax	18,200	(10,500)	39,200	7,400
Net realized and unrealized investment gains (losses)	28,100	(15,000)	57,600	10,000
Short-term investments, at amortized cost (which approximates fair value)	421,800		$ 421,800		211,200
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)			7,100	9,000
Amortization/Accretion			100	0
Purchases			1,449,700	495,400
Sales			(2,043,900)	(604,200)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2016	565,200	450,800	565,200	450,800
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)			38,300	5,900
Amortization/Accretion			(8,100)	(9,900)
Purchases			2,076,900	734,300
Sales			(577,400)	(616,000)
Transfers in			0	34,900
Transfers out			0	0
Balance at June 30, 2016	4,061,100	2,522,100	4,061,100	2,522,100
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)			1,300	0
Amortization/Accretion			0	0
Purchases			84,700	35,300
Sales			(2,800)
Transfers in			0	0
Transfers out			0	(34,900)
Balance at June 30, 2016	153,200	76,800	153,200	76,800
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	6,600
Amortization/Accretion			0	0
Purchases			0	0
Sales			0	(13,100)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2016	0	33,000	0	33,000
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			169,500	126,900
Total realized and unrealized gains (losses)			12,900	(6,800)
Amortization/Accretion			0	0
Purchases			2,100	22,500
Sales			0	(2,300)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2016	184,500	140,300	184,500	140,300
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	177,300
Total realized and unrealized gains (losses)			(2,000)	(3,600)
Amortization/Accretion			0	0
Purchases			9,100	6,200
Sales			(7,100)	(23,300)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2016	127,800	156,600	127,800	156,600
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 June 30, 2016	5,386,100		5,386,100
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 June 30, 2016	987,300		987,300
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 June 30, 2016	4,061,100		4,061,100
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 June 30, 2016	337,700		337,700
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 June 30, 2016	4,372,000		4,372,000
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 June 30, 2016	252,900		252,900
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 June 30, 2016	3,965,900		3,965,900
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 June 30, 2016	153,200		153,200
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 June 30, 2016	184,500		184,500
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			0
Balance at June 30, 2016	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]
Balance at January 1, 2016			0
Balance at June 30, 2016	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]
Balance at January 1, 2016			169,500
Balance at June 30, 2016	184,500		184,500
Carrying value of investment accounted for using the equity method	3,500	3,900	3,500	3,900	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 June 30, 2016	422,100		422,100
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 June 30, 2016	422,100		422,100
Short-term investments, at amortized cost (which approximates fair value)		401,300		401,300	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 June 30, 2016	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]
Balance at January 1, 2016			0
Balance at June 30, 2016	0		0
Discontinued Operations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains (losses)	7,300	(46,100)	(1,500)	32,300
Star & Shield Insurance Exchange | Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments, at amortized cost (which approximates fair value)	300	1,100	300	1,100	100	1,900
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 (losses)				900
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	13,400	15,800	13,400	15,800	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	13,400		13,400		14,700
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities Debt Reclassified to Assets Held-for-sale	$ 9,200	$ 9,100	$ 9,200	$ 9,100	$ 9,500	$ 10,100