Summary of Significant Accounting Policies (Details 6) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended																								12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basic:
Net income for the period	$ 30,068		$ 25,357		$ 20,452		$ 28,366		$ 34,287		$ 23,599		$ 25,995		$ 27,012		$ 31,826		$ 32,266		$ 13,103		$ 26,671		$ 104,243	$ 110,893	$ 103,866
Weighted average number of common shares during the period (in thousands)	41,748	[1]	41,514	[1]	41,432	[1]	41,180	[1]	40,818	[1]	40,001	[1]	39,768	[1]	39,527	[1]	39,339	[1]	39,381	[1]	39,544	[1]	39,794	[1]	41,646,281	40,376,562	39,513,540
Net income per share - basic	$ 0.72		$ 0.61		$ 0.49		$ 0.69		$ 0.84		$ 0.59		$ 0.65		$ 0.68		$ 0.81		$ 0.82		$ 0.33		$ 0.67		$ 2.50	$ 2.75	$ 2.63
Diluted:
Net income for the period	$ 30,068		$ 25,357		$ 20,452		$ 28,366		$ 34,287		$ 23,599		$ 25,995		$ 27,012		$ 31,826		$ 32,266		$ 13,103		$ 26,671		$ 104,243	$ 110,893	$ 103,866
Weighted average number of common shares during the period (in thousands)	41,748	[1]	41,514	[1]	41,432	[1]	41,180	[1]	40,818	[1]	40,001	[1]	39,768	[1]	39,527	[1]	39,339	[1]	39,381	[1]	39,544	[1]	39,794	[1]	41,646,281	40,376,562	39,513,540
Weighted average number of common equivalent shares to reflect the dilutive effect of common stock equivalent securities
Stock options																									137,000	211,000	222,000
Common stock units related to deferred compensation for Directors																									0	0	112,000
Common stock units related to deferred compensation for employees																									70,000	112,000	116,000
Restricted common stock units related to incentive compensation																									378,000	933,000	1,424,000
Total common and common equivalent shares adjusted to calculate diluted earnings per share (in thousands)	42,362	[1]	42,319	[1]	42,310	[1]	42,259	[1]	42,205	[1]	41,732	[1]	41,395	[1]	41,088	[1]	41,272	[1]	41,138	[1]	41,304	[1]	41,546	[1]	42,230,559	41,633,240	41,388,368
Net income per share - diluted	$ 0.71		$ 0.60		$ 0.48		$ 0.67		$ 0.81		$ 0.57		$ 0.63		$ 0.66		$ 0.77		$ 0.78		$ 0.32		$ 0.64		$ 2.47	$ 2.66	$ 2.51

[1] Rounded to thousands.