Description of the Business and Summary of Significant Accounting Policies - Income Per Share (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
Net Income (Loss) Available to Common Stockholders, Diluted [Abstract]
Net income									$ 264,670	$ 176,100	$ 159,366
Net income - assuming dilution									$ 264,670	$ 176,100	$ 159,366
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted average shares outstanding - basic (in shares)									51,294	51,147	51,009
Effect of dilutive stock-based compensation (in shares)									213	224	303
Weighted average shares outstanding - diluted (in shares)									51,507	51,371	51,312
Earnings Per Share, Diluted [Abstract]
Net income, per share (in dollars per share)	$ 3.10	$ 0.44	$ 1.08	$ 0.53	$ 1.41	$ 0.47	$ 0.92	$ 0.64	$ 5.16	$ 3.44	$ 3.12
Net income, per share - assuming dilution (in dollars per share)	$ 3.09	$ 0.44	$ 1.08	$ 0.53	$ 1.41	$ 0.46	$ 0.92	$ 0.63	$ 5.14	$ 3.43	$ 3.11