Investments in Unconsolidated Affiliates (Symetra Summary of Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended							9 Months Ended	12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments in and Advances to Affiliates [Line Items]
Total investments				$ 2,714.4					$ 3,380.7	$ 2,714.4
Assets				6,520.2					3,659.2	6,520.2
Long-term debt				12.7					23.8	12.7
Liabilities				2,804.2					298.4	2,804.2
Common shareholders’ equity				3,716.0					3,360.8	3,716.0
Earned insurance premiums									10.4	13.4	$ 12.0
Net investment income									56.0	32.1	10.9
Revenues	$ 87.5	$ 83.5	$ 88.8	(5.5)	$ 56.8	$ 48.4	$ 58.0		373.8	157.7	440.0
Expenses	$ 79.1	$ 85.7	$ 92.4	$ 69.0	$ 70.6	$ 74.1	$ 91.3		366.0	305.0	310.8
Net income									593.1	409.0	276.7
Comprehensive income (loss)									$ 630.7	$ 547.4	$ 187.4
Symetra
Investments in and Advances to Affiliates [Line Items]
Total investments								$ 32,409.2
Separate account assets								885.9
Assets								34,962.8
Policyholder liabilities								29,492.0
Long-term debt								697.5
Separate account liabilities								885.9
Liabilities								31,836.7
Common shareholders’ equity								3,126.1
Earned insurance premiums								539.3
Net investment income								994.3
Revenues								1,605.9
Policy benefits								1,143.7
Expenses								1,543.6
Net income								89.6
Comprehensive income (loss)								$ (234.1)