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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income available to common stockholders	$ 262,856	$ 267,281	$ 317,987	$ 288,732	$ 266,264	$ 274,817	$ 237,798	$ 194,961	$ 1,136,856	$ 973,840	$ 490,506
Denominator:
Weighted average common shares outstanding-basic (in shares)									51,915	53,078	52,588
Denominator for diluted calculation (in shares)									52,311	53,772	53,306
Earnings per common share:
Basic (usd per share)	$ 5.10	$ 5.19	$ 6.12	$ 5.49	$ 5.01	$ 5.16	$ 4.48	$ 3.69	$ 21.90	$ 18.35	$ 9.33
Diluted (usd per share)	$ 5.06	$ 5.15	$ 6.08	$ 5.44	$ 4.96	$ 5.10	$ 4.42	$ 3.63	$ 21.73	$ 18.11	$ 9.20
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)									227	377	385
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)									169	317	333