Consolidated Statements of Financial Position (Parenthetical) - USD ($) shares in Millions, $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fixed maturities, available-for-sale	$ 58,682.7	$ 59,388.4
Equity securities	1,927.5	1,866.6
Real estate	1,701.1	1,736.7
Other investments	3,692.2	3,586.2
Other investments measured at fair value under fair value option	17.9	61.0
Separate account assets	158,819.2	159,272.7
Contractholder funds	38,319.7	38,082.5
Separate account liabilities	158,819.2	159,272.7
Other liabilities	5,790.8	5,294.1
Redeemable noncontrolling interest	$ 127.8	$ 101.3
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	2,500.0	2,500.0
Common stock, issued (in shares)	476.0	474.1
Common stock, outstanding (in shares)	287.7	289.0
Treasury stock (in shares)	188.3	185.1
Aggregate consolidated variable interest entities
Fixed maturities, available-for-sale	$ 276.1	$ 268.0
Equity securities	821.8	811.4
Real estate	355.7	370.3
Other investments	211.2	139.8
Separate account assets	42,338.6	41,540.8
Contractholder funds	385.8	380.6
Separate account liabilities	42,338.6	41,540.8
Other liabilities	288.7	270.2
Redeemable noncontrolling interest	$ 65.3	$ 52.4