Stockholders' Equity and EPS - Reconciliation of Basic EPS to Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Numerator:
Net income available to common stockholders	$ 266,264	$ 274,817	$ 237,798	$ 194,961	$ 117,210	$ 148,620	$ 123,193	$ 101,483	$ 973,840	[1]	$ 490,506	[1]	$ 382,685
Denominator:
Weighted average common shares outstanding-basic (in shares)									53,078		52,588		51,915
Denominator for diluted calculation (in shares)									53,772		53,306		52,349
Earnings per common share:
Basic (usd per share)	$ 5.01	$ 5.16	$ 4.48	$ 3.69	$ 2.22	$ 2.82	$ 2.34	$ 1.94	$ 18.35	[1]	$ 9.33	[1]	$ 7.37
Diluted (usd per share)	$ 4.96	$ 5.10	$ 4.42	$ 3.63	$ 2.19	$ 2.79	$ 2.32	$ 1.91	$ 18.11	[1]	$ 9.20	[1]	$ 7.31
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)									377		385		254
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)									317		333		180

[1] Included in income tax expense, net income available to common stockholders, earnings per common share-basic and earnings for common share-diluted, for the years ended December 31, 2018 and 2017, are tax benefits recognized associated with the adoption of Accounting Standards Update ("ASU") 2016-09, Improvements to Employee Share-Based Payment Accounting in the first quarter of 2017. This guidance was adopted on a prospective basis with no change to prior period amounts.