Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net income	$ 180.9	$ 654.8	$ 859.1	$ 865.2	$ 506.7	$ 630.9	$ 731.1	$ 701.0	$ 2,560.0	$ 2,569.7	$ 2,489.7
Other comprehensive (loss) income, net of tax:
Change in net unrealized gains/losses on investments									(384.3)	118.6	(294.7)
Change in non-credit component of other-than-temporary impairment losses on investments									(5.6)	(3.9)	1.7
Change in net unrealized gains/losses on cash flow hedges									(45.2)	(3.6)	3.0
Change in net periodic pension and postretirement benefits									(26.0)	(118.1)	172.7
Foreign currency translation adjustments									(3.4)	(4.3)	1.4
Other comprehensive loss									(464.5)	(11.3)	(115.9)
Total comprehensive income									2,095.5	2,558.4	2,373.8
Parent Company [Member]
Net income									2,560.0	2,569.7	2,489.7
Other comprehensive (loss) income, net of tax:
Change in net unrealized gains/losses on investments									(384.3)	118.6	(294.7)
Change in non-credit component of other-than-temporary impairment losses on investments									(5.6)	(3.9)	1.7
Change in net unrealized gains/losses on cash flow hedges									(45.2)	(3.6)	3.0
Change in net periodic pension and postretirement benefits									(26.0)	(118.1)	172.7
Foreign currency translation adjustments									(3.4)	(4.3)	1.4
Other comprehensive loss									(464.5)	(11.3)	(115.9)
Total comprehensive income									$ 2,095.5	$ 2,558.4	$ 2,373.8