Consolidated Statements of Financial Position (Parenthetical) - USD ($) shares in Millions, $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fixed maturities, available-for-sale	$ 59,311.7	$ 59,388.4
Equity securities	1,869.3	1,866.6
Real estate	1,728.9	1,736.7
Other investments	3,634.9	3,586.2
Other investments measured at fair value under fair value option	21.1	61.0
Separate account assets	156,989.0	159,272.7
Contractholder funds	39,128.7	38,082.5
Long-term debt	3,219.9	3,178.4
Separate account liabilities	156,989.0	159,272.7
Other liabilities	5,708.4	5,294.1
Redeemable noncontrolling interest	$ 138.9	$ 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)	476.3	474.1
Common stock, outstanding (in shares)	284.6	289.0
Treasury stock (in shares)	191.7	185.1
Aggregate consolidated variable interest entities
Fixed maturities, available-for-sale	$ 242.7	$ 268.0
Equity securities	791.3	811.4
Real estate	334.6	370.3
Other investments	205.2	139.8
Separate account assets	39,407.5	41,540.8
Contractholder funds	387.3	380.6
Long-term debt	37.1	2.8
Separate account liabilities	39,407.5	41,540.8
Other liabilities	259.1	270.2
Redeemable noncontrolling interest	$ 66.3	$ 52.4