Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income (numerator for diluted EPS)	$ 53,513	$ 65,133
Less: Preference stock dividend	8	9
Income attributable to common stockholders (numerator for basic EPS)	$ 53,505	$ 65,124
Denominator:
Weighted-average shares used in basic EPS (in shares)	186,863	185,982
Employee stock plans (in shares)	1,312	893
Weighted-average shares used in diluted EPS (in shares)	188,175	186,875
Basic earnings per share
Basic (in dollars per share)	$ 0.29	$ 0.35
Diluted earnings per share
Diluted (in dollars per share)	$ 0.28	$ 0.35
Anti-dilutive shares not used in calculating diluted weighted-average shares (in shares)	11,636	11,176