Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net income	$ 368.4	$ 617.8	$ 780.6	$ 703.0	$ 180.9	$ 654.8	$ 859.1	$ 865.2	$ 2,469.8	$ 2,560.0	$ 2,569.7
Other comprehensive income (loss), net of tax:
Change in net unrealized gains/losses on investments									117.9	(384.3)	118.6
Change in non-credit component of other-than-ternporary impairment losses on investments									5.4	(5.6)	(3.9)
Change in net unrealized gains/losses on cash flow hedges									(87.3)	(45.2)	(3.6)
Change in net periodic pension and postretirement costs									(13.4)	(26.0)	(118.1)
Foreign currency translation adjustments									2.1	(3.4)	(4.3)
Other comprehensive income (loss)									24.7	(464.5)	(11.3)
Total comprehensive income									2,494.5	2,095.5	2,558.4
Parent Company [Member]
Net income									2,469.8	2,560.0	2,569.7
Other comprehensive income (loss), net of tax:
Change in net unrealized gains/losses on investments									117.9	(384.3)	118.6
Change in non-credit component of other-than-ternporary impairment losses on investments									5.4	(5.6)	(3.9)
Change in net unrealized gains/losses on cash flow hedges									(87.3)	(45.2)	(3.6)
Change in net periodic pension and postretirement costs									(13.4)	(26.0)	(118.1)
Foreign currency translation adjustments									2.1	(3.4)	(4.3)
Other comprehensive income (loss)									24.7	(464.5)	(11.3)
Total comprehensive income									$ 2,494.5	$ 2,095.5	$ 2,558.4