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Selected Quarterly Data - Summary of Selected Quarterly Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
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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
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenues | $ 112,137 us-gaap_Revenues | $ 117,841 us-gaap_Revenues | $ 112,749 us-gaap_Revenues | $ 107,871 us-gaap_Revenues | $ 106,498 us-gaap_Revenues | $ 100,409 us-gaap_Revenues | $ 96,140 us-gaap_Revenues | $ 86,168 us-gaap_Revenues | $ 450,598 us-gaap_Revenues | $ 389,215 us-gaap_Revenues | $ 280,086 us-gaap_Revenues
Operating Income | 36,665 us-gaap_OperatingIncomeLoss | 38,927 us-gaap_OperatingIncomeLoss | 22,502 us-gaap_OperatingIncomeLoss | 32,626 us-gaap_OperatingIncomeLoss | 33,892 us-gaap_OperatingIncomeLoss | 31,630 us-gaap_OperatingIncomeLoss | 26,882 us-gaap_OperatingIncomeLoss | 21,100 us-gaap_OperatingIncomeLoss | 130,720 us-gaap_OperatingIncomeLoss | 113,504 us-gaap_OperatingIncomeLoss | 60,445 us-gaap_OperatingIncomeLoss
Net Income Attributable to Common Stockholders | $ 18,879 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 37,340 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 19,543 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 21,938 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 24,756 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 21,089 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 15,385 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 13,960 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 97,700 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 75,190 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | $ 37,608 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Earnings per share-Basic | $ 2.09 us-gaap_EarningsPerShareBasic | $ 4.10 us-gaap_EarningsPerShareBasic | $ 2.14 us-gaap_EarningsPerShareBasic | $ 2.41 us-gaap_EarningsPerShareBasic | $ 2.72 us-gaap_EarningsPerShareBasic | $ 2.64 us-gaap_EarningsPerShareBasic | $ 1.97 us-gaap_EarningsPerShareBasic | $ 1.79 us-gaap_EarningsPerShareBasic | $ 10.75 us-gaap_EarningsPerShareBasic | $ 9.18 us-gaap_EarningsPerShareBasic | $ 4.87 us-gaap_EarningsPerShareBasic
Earnings per share-Diluted | $ 2.05 us-gaap_EarningsPerShareDiluted | $ 4.02 us-gaap_EarningsPerShareDiluted | $ 2.10 us-gaap_EarningsPerShareDiluted | $ 2.34 us-gaap_EarningsPerShareDiluted | $ 2.65 us-gaap_EarningsPerShareDiluted | $ 2.56 us-gaap_EarningsPerShareDiluted | $ 1.91 us-gaap_EarningsPerShareDiluted | $ 1.73 us-gaap_EarningsPerShareDiluted | $ 10.51 us-gaap_EarningsPerShareDiluted | $ 8.92 us-gaap_EarningsPerShareDiluted | $ 4.66 us-gaap_EarningsPerShareDiluted

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