Net Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Earnings Per Share [Abstract]
Net (loss) earnings attributable to common stockholders	$ (320.0)	$ (30.0)	$ 47.0	$ 26.0	$ 27.0	$ 90.0	$ 352.0	$ 231.0	$ (277.0)	$ 700.0	$ 1,390.0
Basic earnings per common share:
Weighted-average common shares outstanding (in shares)									233.1	235.3	255.9
Net earnings attributable to common stockholders (in dollars per share)	$ (1.38)	$ (0.13)	$ 0.20	$ 0.11	$ 0.11	$ 0.39	$ 1.50	$ 0.96	$ (1.19)	$ 2.97	$ 5.43
Diluted earnings per common share:
Weighted-average common shares outstanding (in shares)									233.1	235.3	255.9
Dilutive common shares—stock options (in shares)									0.0	0.8	0.8
Diluted weighted-average shares outstanding (in shares)									233.1	236.1	256.7
Net earnings attributable to common stockholders (in dollars per share)	$ (1.38)	$ (0.13)	$ 0.20	$ 0.11	$ 0.11	$ 0.39	$ 1.49	$ 0.96	$ (1.19)	$ 2.96	$ 5.42
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									4.9	1.6	0.7