Earnings per Share Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income									$ 72,081	$ 56,609	$ 51,763
Preferred dividends									(3,656)	(4,875)	(4,875)
Excess of fair value over book value of preferred shares redeemed									(2,257)	0	0
Net Income Available to Common Stockholders	$ 15,882	$ 18,852	$ 15,770	$ 15,664	$ 19,343	$ 9,724	$ 14,697	$ 7,970	66,168	51,734	46,888
Diluted income available to common shareholders									$ 69,387	$ 55,304	$ 50,408
Basic Weighted Average Shares Outstanding	27,736	25,581	24,990	24,738	24,671	24,669	24,669	24,657	25,769	24,666	24,575
Stock option awards									342	216	237
Deferred compensation awards									221	149	150
Diluted Weighted Average Shares Outstanding	31,172	30,675	30,619	30,329	30,166	30,139	30,077	30,032	30,688	30,116	30,047
Earnings Per Share, Basic	$ 0.57	$ 0.74	$ 0.63	$ 0.63	$ 0.78	$ 0.39	$ 0.60	$ 0.32	$ 2.57	$ 2.10	$ 1.91
Earnings Per Share, Diluted	$ 0.53	$ 0.64	$ 0.55	$ 0.55	$ 0.67	$ 0.35	$ 0.52	$ 0.30	$ 2.26	$ 1.84	$ 1.68
Anti-dilutive stock equivalent awards not included in the calculation of diluted loss per share									23	1,273	1,447
2018 Convertible Senior Notes [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Interest on convertible senior subordinated notes									$ 2,113	$ 2,050	$ 2,021
Convertible senior subordinated notes potential shares									2,669	2,669	2,669
2017 Convertible Senior Notes [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Interest on convertible senior subordinated notes									$ 1,106	$ 1,520	$ 1,499
Convertible senior subordinated notes potential shares									1,687	2,416	2,416