Earnings Per Share (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
Earnings Per Share Reconciliation [Abstract]
Net income	$ 50,614	$ 30,723	$ 28,479	$ 18,871	$ 49,871	$ 32,358	$ 31,376	$ 25,820	$ 128,687	$ 139,425	$ 152,439
Net income available to participating securities									1,885	1,792	2,242
Net earnings available to common shareholders									$ 126,802	$ 137,633	$ 150,197
Basic (in shares)	47,967,014	48,009,953	47,951,326	47,873,007	47,721,700	47,609,819	47,547,118	47,463,231	47,950,746	47,586,242	47,668,357
Dilutive effect of equity incentive plans (in shares)									146,906	228,166	201,660
Dilutive effect of convertible debt (in shares)									830,139	854,619	93,209
Diluted (in shares)	48,505,023	49,279,217	49,218,521	48,706,717	48,437,580	48,521,082	48,948,315	48,766,031	48,927,791	48,669,027	47,963,226
Basic (in dollars per share)	$ 1.04	$ 0.63	$ 0.59	$ 0.39	$ 1.03	$ 0.67	$ 0.65	$ 0.54	$ 2.64	$ 2.89	$ 3.15
Earnings Per Share, Diluted	$ 1.03	$ 0.61	$ 0.57	$ 0.38	$ 1.02	$ 0.66	$ 0.63	$ 0.52	$ 2.59	$ 2.83	$ 3.13
Share options excluded from the computation of diluted earnings per share									0