Summary of Significant Accounting Policies - Weighted Average Shares Outstanding (Details) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share Basic And Diluted Other Disclosures [Abstract]
Weighted average shares outstanding - basic	45,754	46,107	46,931	44,200	30,713	30,695	30,680	30,658	45,754	30,687	31,100
Dilutive effect of equity incentive plan instruments									1,395	977	991
Weighted average shares outstanding - diluted	47,149	47,452	48,195	45,419	31,798	31,698	31,620	31,538	47,149	31,664	32,091