Fair Value Measurements - Level 3 Inputs Reconciliation (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Level 1 to level 3 Transfers, Amount	$ 0	$ 0
Fair Value, Level 3 to level 1 Transfers, Amount	0	0
Fair Value, Level 1 to Level 2 Activity Transfers, Amount	0	0
Fair Value, Level 2 to Level 1 Activity Transfers, Amount	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	626,700,000	520,400,000
Included in Consolidated Statement of Income	0	(10,000,000)
Included in Other Comprehensive Income	6,800,000	1,000,000
Purchases	347,400,000	270,100,000
Settlements	(172,100,000)	(151,400,000)
Sales	(96,600,000)	(2,800,000)
Transfers Into Level 3	3,600,000	8,200,000
Transfers out of Level 3	(1,900,000)	(8,800,000)
Balance at End of Year	713,900,000	626,700,000
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	364,100,000	361,000,000
Included in Consolidated Statement of Income	(3,400,000)	(7,900,000)
Included in Other Comprehensive Income	5,100,000	(7,800,000)
Purchases	201,400,000	145,100,000
Settlements	(126,400,000)	(123,200,000)
Sales	(79,000,000)	(100,000)
Transfers Into Level 3	0	5,800,000
Transfers out of Level 3	(1,200,000)	(8,800,000)
Balance at End of Year	360,600,000	364,100,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	7,400,000	4,700,000
Included in Consolidated Statement of Income	0	200,000
Included in Other Comprehensive Income	(100,000)	(1,300,000)
Purchases	0	5,100,000
Settlements	(600,000)	(1,300,000)
Sales	0	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Year	6,700,000	7,400,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	44,700,000	0
Included in Consolidated Statement of Income	200,000	200,000
Included in Other Comprehensive Income	(1,000,000)	400,000
Purchases	26,100,000	44,100,000
Settlements	0	0
Sales	(5,600,000)	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Year	64,400,000	44,700,000
Other Mortgage and Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	4,500,000	100,000
Included in Consolidated Statement of Income	100,000	100,000
Included in Other Comprehensive Income	100,000	(500,000)
Purchases	0	4,500,000
Settlements	(800,000)	(100,000)
Sales	0	(2,000,000)
Transfers Into Level 3	0	2,400,000
Transfers out of Level 3	0	0
Balance at End of Year	3,900,000	4,500,000
Preferred and Common Stock [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	32,100,000	13,300,000
Included in Consolidated Statement of Income	1,100,000	(1,000,000)
Included in Other Comprehensive Income	500,000	4,600,000
Purchases	8,900,000	15,900,000
Settlements	(4,500,000)	0
Sales	(2,200,000)	(700,000)
Transfers Into Level 3	3,600,000	0
Transfers out of Level 3	(700,000)	0
Balance at End of Year	38,800,000	32,100,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	173,900,000	141,300,000
Included in Consolidated Statement of Income	2,700,000	(1,600,000)
Included in Other Comprehensive Income	2,200,000	5,600,000
Purchases	53,000,000	55,400,000
Settlements	(39,800,000)	(26,800,000)
Sales	(5,800,000)	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Year	186,200,000	173,900,000
Fair Value Option Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	0
Included in Consolidated Statement of Income	(700,000)
Included in Other Comprehensive Income	0
Purchases	58,000,000
Settlements	0
Sales	(4,000,000)
Transfers Into Level 3	0
Transfers out of Level 3	0
Balance at End of Year	$ 53,300,000