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
Numerator:
Net income	$ 14,127us-gaap_NetIncomeLoss	$ 8,404us-gaap_NetIncomeLoss	$ 7,741us-gaap_NetIncomeLoss	$ 6,272us-gaap_NetIncomeLoss	$ 10,022us-gaap_NetIncomeLoss	$ 8,315us-gaap_NetIncomeLoss	$ 9,462us-gaap_NetIncomeLoss	$ 6,960us-gaap_NetIncomeLoss	$ 36,544us-gaap_NetIncomeLoss	$ 34,759us-gaap_NetIncomeLoss	$ 34,262us-gaap_NetIncomeLoss
Less: Net income allocated to participating securities									(277)	(336)	(282)
Net income available to common shareholders									$ 36,267	$ 34,423	$ 33,980
Denominator:
Weighted average common shares outstanding									20,922	20,821	20,391
Weighted average dilutive securities									544	610	781
Weighted average common shares outstanding, diluted									21,466	21,431	21,172
Income per common share, basic (in dollars per share)	$ 0.68us-gaap_EarningsPerShareBasic	$ 0.40us-gaap_EarningsPerShareBasic	$ 0.36us-gaap_EarningsPerShareBasic	$ 0.30us-gaap_EarningsPerShareBasic	$ 0.47us-gaap_EarningsPerShareBasic	$ 0.39us-gaap_EarningsPerShareBasic	$ 0.45us-gaap_EarningsPerShareBasic	$ 0.33us-gaap_EarningsPerShareBasic	$ 1.73us-gaap_EarningsPerShareBasic	$ 1.65us-gaap_EarningsPerShareBasic	$ 1.67us-gaap_EarningsPerShareBasic
Income per common share, diluted (in dollars per share)	$ 0.66us-gaap_EarningsPerShareDiluted	$ 0.39us-gaap_EarningsPerShareDiluted	$ 0.36us-gaap_EarningsPerShareDiluted	$ 0.29us-gaap_EarningsPerShareDiluted	$ 0.46us-gaap_EarningsPerShareDiluted	$ 0.38us-gaap_EarningsPerShareDiluted	$ 0.44us-gaap_EarningsPerShareDiluted	$ 0.32us-gaap_EarningsPerShareDiluted	$ 1.69us-gaap_EarningsPerShareDiluted	$ 1.61us-gaap_EarningsPerShareDiluted	$ 1.60us-gaap_EarningsPerShareDiluted

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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