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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income available to common stockholders	$ 388,316	$ 441,713	$ 228,935	$ 132,253	$ 262,856	$ 267,281	$ 317,987	$ 288,732	$ 1,191,217	$ 1,136,856	$ 973,840
Denominator:
Weighted average common shares outstanding-basic (in shares)									51,685	51,915	53,078
Denominator for diluted calculation (in shares)									52,084	52,311	53,772
Earnings per common share:
Basic (usd per share)	$ 7.49	$ 8.53	$ 4.44	$ 2.56	$ 5.10	$ 5.19	$ 6.12	$ 5.49	$ 23.05	$ 21.90	$ 18.35
Diluted (usd per share)	$ 7.40	$ 8.47	$ 4.42	$ 2.55	$ 5.06	$ 5.15	$ 6.08	$ 5.44	$ 22.87	$ 21.73	$ 18.11
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)									151	227	377
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)									248	169	317