Net Earnings (Loss) Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net earnings (loss)	$ 79.0	$ 787.5	$ (7.2)	$ 848.4
Distributed and allocated undistributed net earnings to unvested restricted stockholders	(0.4)	(5.0)	(0.4)	(5.3)
Distributed and allocated undistributed net earnings (loss)	78.6	782.5	(7.6)	843.1
Distributed net earnings - dividends paid to common stockholders	(25.0)	(30.1)	(76.9)	(91.5)
Allocation of undistributed net earnings (loss) to common stockholders	$ 53.6	$ 752.4	$ (84.5)	$ 751.6
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	157.2	186.9	160.8	190.9
Basic net earnings per common share:
Distributed net earnings (in dollars per share)	$ 0.16	$ 0.16	$ 0.48	$ 0.48
Allocated undistributed net earnings (loss) to common stockholders (in dollars per share)	0.34	4.03	(0.53)	3.94
Net earnings (loss) per common share - basic (in dollars per share)	$ 0.50	$ 4.19	$ (0.05)	$ 4.42
Numerator:
Distributed and allocated undistributed net earnings (loss)	$ 78.6	$ 782.5	$ (7.6)	$ 843.1
Add: Allocated undistributed net earnings to unvested restricted stockholders	0.3	4.8	0.0	4.8
Less: Undistributed net earnings reallocated to unvested restricted stockholders	(0.3)	(4.8)	0.0	(4.8)
Net earnings (loss) available to common stockholders - diluted	$ 78.6	$ 782.5	$ (7.6)	$ 843.1
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	157.2	186.9	160.8	190.9
Effect of contingently issuable shares (in shares)	0.1	0.7	0.0	0.7
Effect of unvested restricted stock units (in shares)	0.3	0.7	0.0	0.7
Weighted average number of common shares outstanding - diluted under two-class (in shares)	157.6	188.3	160.8	192.3
Effect of unvested restricted stock - participating security (in shares)	0.4	0.6	0.0	0.6
Weighted average number of common shares outstanding - diluted under treasury stock (in shares)	158.0	188.9	160.8	192.9
Diluted net earnings (loss) per common share (in dollars per share)	$ 0.50	$ 4.15	$ (0.05)	$ 4.37