Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Document Period End Date			Jun. 30, 2018
Net Income (Loss) Attributable to Parent [Abstract]
Net income attributable to common stockholders	$ 103,239	$ 68,481	$ 185,227	$ 243,183
After tax interest on convertible debt	15	13	30	25
Net income — diluted	$ 103,254	$ 68,494	$ 185,257	$ 243,208
Weighted average shares (denominator):
Weighted average shares — basic	(182,637)	(187,597)	(183,877)	(187,927)
Convertible debt	121	121	121	121
Weighted average shares — diluted	187,188	192,138	188,547	192,511
Net income attributable to common stockholders, basic	$ 0.57	$ 0.37	$ 1.01	$ 1.29
Net income attributable to common stockholders, diluted	$ 0.55	$ 0.36	$ 0.98	$ 1.26
Equity Option [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Stock options	4,265	4,333	4,391	4,389
Restricted Stock Units (RSUs) [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Stock options	165	87	158	74
Equity Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1,155	1,523	868	1,212