Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Calculation of basic and diluted earnings per share
Net income	$ 317,401	$ 74,434
Basic weighted-average common shares outstanding	111,696	111,258
Dilutive effect of non-vested RSUs and contingent PSUs	1,183	71
Dilutive effect of Senior Convertible Notes	0	0
Diluted weighted-average common shares outstanding	112,879	111,329
Basic net income per common share	$ 2.84	$ 0.67
Diluted net income per common share	2.81	$ 0.67
1.50% Senior Convertible Notes Due 2021 [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Debt Instrument, Convertible, Conversion Price	$ 40.50