Summary of Significant Accounting Policies (Details 6) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended																								12 Months Ended
	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, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Basic:
Net income per share - basic	$ 0.84		$ 0.59		$ 0.65		$ 0.68		$ 0.81		$ 0.82		$ 0.33		$ 0.67		$ 0.82		$ 0.59		$ (0.30)		$ 0.65		$ 2.75	$ 2.63	$ 1.77
Net income for the period	$ 34,287		$ 23,599		$ 25,995		$ 27,012		$ 31,826		$ 32,266		$ 13,103		$ 26,671		$ 32,913		$ 23,637		$ (11,851)		$ 25,807		$ 110,893	$ 103,866	$ 70,506
Weighted average number of common shares during the period (in thousands)	40,818	[1]	40,001	[1]	39,768	[1]	39,527	[1]	39,339	[1]	39,381	[1]	39,544	[1]	39,794	[1]	39,900	[1]	39,919	[1]	39,893	[1]	39,749	[1]	40,376,562	39,513,540	39,865,815
Diluted:
Net income	$ 34,287		$ 23,599		$ 25,995		$ 27,012		$ 31,826		$ 32,266		$ 13,103		$ 26,671		$ 32,913		$ 23,637		$ (11,851)		$ 25,807		$ 110,893	$ 103,866	$ 70,506
Weighted average number of common shares during the period (in thousands)	42,205	[1]	41,732	[1]	41,395	[1]	41,088	[1]	41,272	[1]	41,138	[1]	41,304	[1]	41,546	[1]	41,414	[1]	41,451	[1]	39,893	[1]	41,699	[1]	41,633,240	41,388,368	41,436,512
Weighted average number of common equivalent shares to reflect the dilutive effect of common stock equivalent securities (in thousands):
Stock options																									211,000	222,000	206,000
Common stock units related to deferred compensation for Directors																									0	112,000	114,000
Common stock units related to deferred compensation for Employees																									112,000	116,000	115,000
Restricted common stock units related to incentive compensation																									933,000	1,424,000	1,136,000
Total common and common equivalent shares adjusted to calculate diluted earnings per share (in thousands)																									41,633,000	41,388,000	41,437,000
Net income per share - diluted	$ 0.81		$ 0.57		$ 0.63		$ 0.66		$ 0.77		$ 0.78		$ 0.32		$ 0.64		$ 0.79		$ 0.57		$ (0.30)		$ 0.62		$ 2.66	$ 2.51	$ 1.70

[1] Rounded to thousands.