Consolidated Statements of Financial Position (Parenthetical) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fixed maturities, available-for-sale	$ 59,388.4	$ 54,846.1
Fixed maturities, trading	566.0	398.4
Equity securities, trading	1,770.6	1,413.4
Real estate	1,736.7	1,368.8
Other investments	3,586.2	3,655.9
Other investments measured at fair value under fair value option	61.0	86.2
Separate account assets	159,272.7	139,832.6
Contractholder funds	38,082.5	37,953.6
Separate account liabilities	159,272.7	139,832.6
Other liabilities	5,294.1	5,783.3
Redeemable noncontrolling interest	$ 101.3	$ 97.5
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)	474.1	469.2
Common stock, outstanding (in shares)	289.0	287.7
Treasury stock (in shares)	185.1	181.5
Aggregate consolidated variable interest entities
Fixed maturities, available-for-sale	$ 268.0	$ 232.5
Fixed maturities, trading	0.0	82.4
Equity securities, trading	811.4	721.9
Real estate	370.3	305.7
Other investments	139.8	89.8
Separate account assets	41,540.8	35,844.1
Contractholder funds	380.6	358.7
Separate account liabilities	41,540.8	35,844.1
Other liabilities	270.2	284.1
Other liabilities measured at fair value under fair value option	0.0	59.9
Redeemable noncontrolling interest	$ 52.4	$ 58.8