Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
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	606,800,000	518,400,000
Included in Consolidated Statement of Income	(26,200,000)	(18,200,000)
Included in Other Comprehensive Income	5,900,000	(7,300,000)
Purchases	244,600,000	296,800,000
Settlements	(101,800,000)	(75,600,000)
Sales	(125,700,000)	(87,300,000)
Transfers Into Level 3	4,100,000	0
Transfers out of Level 3	(26,000,000)	(20,000,000)
Balance at End of Year	581,700,000	606,800,000
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	436,300,000	360,600,000
Included in Consolidated Statement of Income	(23,000,000)	(9,400,000)
Included in Other Comprehensive Income	900,000	(7,400,000)
Purchases	203,700,000	241,600,000
Settlements	(85,400,000)	(65,500,000)
Sales	(114,500,000)	(73,600,000)
Transfers Into Level 3	0	0
Transfers out of Level 3	(14,800,000)	(10,000,000)
Balance at End of Year	403,200,000	436,300,000
States and Political Subdivisions [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	(300,000)
Included in Other Comprehensive Income	0
Purchases	0
Settlements	0
Sales	0
Transfers Into Level 3	4,100,000
Transfers out of Level 3	0
Balance at End of Year	3,800,000	0
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	3,800,000	6,700,000
Included in Consolidated Statement of Income	0	(1,600,000)
Included in Other Comprehensive Income	0	(700,000)
Purchases	0	0
Settlements	(3,200,000)	(600,000)
Sales	0	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Year	600,000	3,800,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	87,300,000	64,400,000
Included in Consolidated Statement of Income	0	600,000
Included in Other Comprehensive Income	4,300,000	(2,200,000)
Purchases	29,100,000	39,500,000
Settlements	(5,000,000)	(7,500,000)
Sales	(1,900,000)	(7,500,000)
Transfers Into Level 3	0	0
Transfers out of Level 3	(10,300,000)	0
Balance at End of Year	103,500,000	87,300,000
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	3,800,000	3,900,000
Included in Consolidated Statement of Income	400,000	0
Included in Other Comprehensive Income	(300,000)	(100,000)
Purchases	0	1,300,000
Settlements	(3,000,000)	(100,000)
Sales	0	(1,200,000)
Transfers Into Level 3	0	0
Transfers out of Level 3	(900,000)	0
Balance at End of Year	0	3,800,000
Preferred and Common Stock [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	30,000,000	38,800,000
Included in Consolidated Statement of Income	(1,700,000)	(2,600,000)
Included in Other Comprehensive Income	3,400,000	1,400,000
Purchases	7,900,000	8,100,000
Settlements	(5,200,000)	(700,000)
Sales	(4,700,000)	(5,000,000)
Transfers Into Level 3	0	0
Transfers out of Level 3	0	(10,000,000)
Balance at End of Year	29,700,000	30,000,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	45,600,000	44,000,000
Included in Consolidated Statement of Income	(1,600,000)	(5,200,000)
Included in Other Comprehensive Income	(2,400,000)	1,700,000
Purchases	3,900,000	6,300,000
Settlements	0	(1,200,000)
Sales	(4,600,000)	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Balance at End of Year	$ 40,900,000	$ 45,600,000