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Stockholders' Equity - Repurchase Activities (Details) (USD $)
 1 Months Ended 12 Months Ended
May 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Equity [Abstract]      
Authorized increase to stock repurchase plan $ 30,000,000core_StockRepurchaseProgramAuthorizedAmountIncrease    
Common stock available for future share repurchases, amount 2,300,000us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1 20,700,000us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1 28,700,000us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1
Number of shares repurchased   175,917us-gaap_TreasuryStockSharesAcquired 253,744us-gaap_TreasuryStockSharesAcquired
Average price per share (in dollars per share)   $ 45.49us-gaap_TreasuryStockAcquiredAverageCostPerShare $ 28.30us-gaap_TreasuryStockAcquiredAverageCostPerShare
Total repurchase costs   $ 8,000,000us-gaap_TreasuryStockValueAcquiredCostMethod $ 7,200,000us-gaap_TreasuryStockValueAcquiredCostMethod