Earnings Per Share (Details) (USD $) In Thousands, except Per 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, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Abstract]
Income from continuing operations for calculating basic and diluted earnings per share	$ 26,941	$ 19,528	$ 16,015	$ 22,954	$ 25,518	$ 23,127	$ 15,654	$ 20,180	$ 85,438	$ 84,479	$ 80,862
Loss from discontinued operations	0	0	0	(272)	(740)	14	9	84	(272)	(633)	(503)
Net income	$ 26,941us-gaap_NetIncomeLoss	$ 19,528us-gaap_NetIncomeLoss	$ 16,015us-gaap_NetIncomeLoss	$ 22,682us-gaap_NetIncomeLoss	$ 24,778us-gaap_NetIncomeLoss	$ 23,141us-gaap_NetIncomeLoss	$ 15,663us-gaap_NetIncomeLoss	$ 20,264us-gaap_NetIncomeLoss	$ 85,166us-gaap_NetIncomeLoss	$ 83,846us-gaap_NetIncomeLoss	$ 80,359us-gaap_NetIncomeLoss
Weighted-average common shares for calculating basic earnings per share									28,671	29,079	28,912
Stock options and nonvested awards									399	495	801
Weighted-average common shares for calculating diluted earnings per share	28,804	28,805	29,341	29,342	29,393	29,353	29,603	29,955	29,070	29,574	29,713
Income from continuing operations (basic) (in dollars per share)									$ 2.98	$ 2.91	$ 2.80
Loss from discontinued operations (basic) (in dollars per share)									$ (0.01)	$ (0.02)	$ (0.02)
Net income per basic share (in dollars per share)									$ 2.97us-gaap_EarningsPerShareBasic	$ 2.89us-gaap_EarningsPerShareBasic	$ 2.78us-gaap_EarningsPerShareBasic
Income from continuing operations (diluted) (in dollars per share)	$ 0.94	$ 0.68	$ 0.55	$ 0.78	$ 0.87	$ 0.79	$ 0.53	$ 0.68	$ 2.94	$ 2.86	$ 2.72
Loss from discontinued operations (diluted) (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ (0.03)	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ (0.02)	$ (0.02)
Net income per diluted share (in dollars per share)	$ 0.94us-gaap_EarningsPerShareDiluted	$ 0.68us-gaap_EarningsPerShareDiluted	$ 0.55us-gaap_EarningsPerShareDiluted	$ 0.77us-gaap_EarningsPerShareDiluted	$ 0.84us-gaap_EarningsPerShareDiluted	$ 0.79us-gaap_EarningsPerShareDiluted	$ 0.53us-gaap_EarningsPerShareDiluted	$ 0.68us-gaap_EarningsPerShareDiluted	$ 2.93us-gaap_EarningsPerShareDiluted	$ 2.84us-gaap_EarningsPerShareDiluted	$ 2.70us-gaap_EarningsPerShareDiluted

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