Summary of Significant Accounting Policies - Income Statement (Details) - USD ($) $ / shares in Units, $ 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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 244,979	$ 291,490	$ 295,441	$ 209,394	$ 221,483	$ 269,930	$ 261,016	$ 188,868	$ 1,041,304	$ 941,297	$ 807,909
Operating expenses									718,623	651,901	621,481
Income before income taxes	40,330	102,443	100,024	30,461	54,781	83,743	76,414	34,279	273,258	249,217	148,140
Income Tax Expense (Benefit)									56,903	126,890	41,428
Net income	$ 31,471	$ 79,959	$ 79,839	$ 25,086	$ (9,816)	$ 57,189	$ 50,685	$ 24,269	$ 216,355	$ 122,327	$ 106,712
Diluted earnings per share (in dollars per share)	$ 0.56	$ 1.41	$ 1.40	$ 0.44	$ (0.17)	$ 1.01	$ 0.89	$ 0.43	$ 3.80	$ 2.15	$ 1.89
As Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues										$ 1,007,356	$ 924,641
Operating expenses										742,891	686,149
Income before income taxes										223,997	199,980
Income Tax Expense (Benefit)										109,104	60,609
Net income										$ 114,893	$ 139,371
Diluted earnings per share (in dollars per share)										$ 2.02	$ 2.46
Adoption of Topic 606 | ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues										$ (66,059)	$ (116,732)
Operating expenses										(90,990)	(64,668)
Income before income taxes										25,220	(51,840)
Income Tax Expense (Benefit)										17,786	(19,181)
Net income										$ 7,434	$ (32,659)
Diluted earnings per share (in dollars per share)										$ 0.13	$ (0.57)