Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
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	713,900,000	626,700,000
Included in Consolidated Statement of Income	(17,900,000)	0
Included in Other Comprehensive Income	(8,700,000)	6,800,000
Purchases	461,400,000	347,400,000
Settlements	(114,400,000)	(172,100,000)
Sales	(87,300,000)	(96,600,000)
Transfers Into Level 3	4,100,000	3,600,000
Transfers out of Level 3	(19,900,000)	(1,900,000)
Balance at End of Year	931,200,000	713,900,000
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	360,600,000	364,100,000
Included in Consolidated Statement of Income	(9,400,000)	(3,400,000)
Included in Other Comprehensive Income	(7,400,000)	5,100,000
Purchases	241,500,000	201,400,000
Settlements	(65,500,000)	(126,400,000)
Sales	(73,600,000)	(79,000,000)
Transfers Into Level 3	0	0
Transfers out of Level 3	(10,000,000)	(1,200,000)
Balance at End of Year	436,200,000	360,600,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	6,700,000	7,400,000
Included in Consolidated Statement of Income	(1,600,000)	0
Included in Other Comprehensive Income	(700,000)	(100,000)
Purchases	0	0
Settlements	(600,000)	(600,000)
Sales	0	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Year	3,800,000	6,700,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	64,400,000	44,700,000
Included in Consolidated Statement of Income	600,000	200,000
Included in Other Comprehensive Income	(2,200,000)	(1,000,000)
Purchases	39,500,000	26,100,000
Settlements	(7,500,000)	0
Sales	(7,500,000)	(5,600,000)
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Year	87,300,000	64,400,000
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	3,900,000	4,500,000
Included in Consolidated Statement of Income	0	100,000
Included in Other Comprehensive Income	(100,000)	100,000
Purchases	1,300,000	0
Settlements	(100,000)	(800,000)
Sales	(1,200,000)	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Year	3,800,000	3,900,000
Preferred and Common Stock [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	38,800,000	32,100,000
Included in Consolidated Statement of Income	(2,600,000)	1,100,000
Included in Other Comprehensive Income	1,400,000	500,000
Purchases	8,100,000	8,900,000
Settlements	(700,000)	(4,500,000)
Sales	(5,000,000)	(2,200,000)
Transfers Into Level 3	0	3,600,000
Transfers out of Level 3	(9,900,000)	(700,000)
Balance at End of Year	30,100,000	38,800,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	186,200,000	173,900,000
Included in Consolidated Statement of Income	(5,100,000)	2,700,000
Included in Other Comprehensive Income	300,000	2,200,000
Purchases	60,000,000	53,000,000
Settlements	(40,000,000)	(39,800,000)
Sales	0	(5,800,000)
Transfers Into Level 3	4,100,000	0
Transfers out of Level 3	0	0
Balance at End of Year	205,500,000	186,200,000
Fair Value Option Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	53,300,000	0
Included in Consolidated Statement of Income	200,000	(700,000)
Included in Other Comprehensive Income	0	0
Purchases	111,000,000	58,000,000
Settlements	0	0
Sales	0	(4,000,000)
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Year	$ 164,500,000	$ 53,300,000