Capital Stock - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended	12 Months Ended
	Mar. 02, 2016	Feb. 26, 2016	Feb. 22, 2016	Feb. 28, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 01, 2014
Incentive Plans
Issuance of common stock (in shares)	6,600,000.0		44,000,000
Share price (in usd per share)			$ 19.675
Sale of common stock, net	$ 129,900	$ 865,700			$ 0	$ 995,279	$ 0
Treasury stock, number of shares authorized to be repurchased					40,000,000.0
Number of shares repurchased					5,000,000.0	0	0
Cost of shares repurchased					$ 123,700
Cumulative number of shares repurchased under a share repurchase program					34,900,000
Treasury stock, cumulative shares retired					20,000,000
Cumulative cost of shares repurchased under a share repurchase program					$ 512,100
Treasury stock reissued					0
Shares held as treasury stock (in shares)					14,935,926	9,892,680
2014 Incentive Plan
Incentive Plans
Number of shares reserved for issuance								18,000,000
Number of shares available for issuance					14,800,000
2014 Incentive Plan | Stock options | Maximum
Incentive Plans
Number of shares reserved for issuance								10,000,000
Subsequent Event
Incentive Plans
Incremental shares approved for repurchase (in shares)				25,000,000
Treasury stock, number of shares authorized to be repurchased				30,100,000