Earnings per Share Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income									$ 51,763	$ 50,789	$ 151,423
Preferred dividends									(4,875)	(4,875)	(3,656)
Excess of fair value over book value of preferred shares redeemed									0	0	(2,190)
Net Income Available to Common Stockholders	$ 12,056	$ 14,352	$ 12,131	$ 8,349	$ 9,767	$ 12,399	$ 12,335	$ 11,413	46,888	45,914	145,577
Net income available to common shareholders									$ 50,408	$ 49,448	$ 150,695
Basic Weighted Average Shares Outstanding	24,649	24,605	24,531	24,514	24,489	24,474	24,470	24,417	24,575	24,463	23,822
Stock option awards									237	222	237
Deferred compensation awards									150	142	123
Diluted Weighted Average Shares Outstanding	30,107	30,067	30,023	29,975	29,944	29,921	29,913	29,870	30,047	29,912	28,763
Earnings Per Share, Basic	$ 0.49	$ 0.58	$ 0.49	$ 0.34	$ 0.40	$ 0.51	$ 0.50	$ 0.47	$ 1.91	$ 1.88	$ 6.11
Earnings Per Share, Diluted	$ 0.43	$ 0.51	$ 0.43	$ 0.31	$ 0.36	$ 0.44	$ 0.44	$ 0.41	$ 1.68	$ 1.65	$ 5.24
Anti-dilutive stock equivalent awards not included in the calculation of diluted loss per share									1,447	1,250	963
2018 Convertible Senior Notes [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Interest on 3.25% convertible senior subordinated notes due 2017									$ 2,021	$ 2,030	$ 2,675
Convertible senior subordinated notes potential shares									2,669	2,669	2,165
2017 Convertible Senior Notes [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Interest on 3.25% convertible senior subordinated notes due 2017									$ 1,499	$ 1,504	$ 2,443
Convertible senior subordinated notes potential shares									2,416	2,416	2,416