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Summary of Activity Under Credit Arrangements, Excluding Acquired Borrowings (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Debt Disclosure [Abstract]      
Total borrowings at December 31 $ 200,000us-gaap_LongTermDebt $ 280,500us-gaap_LongTermDebt $ 100,000us-gaap_LongTermDebt
Average outstanding borrowings during year 361,715us-gaap_LineOfCreditFacilityAverageOutstandingAmount 138,400us-gaap_LineOfCreditFacilityAverageOutstandingAmount 100,000us-gaap_LineOfCreditFacilityAverageOutstandingAmount
Maximum outstanding borrowings during year $ 500,000us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod $ 411,000us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod $ 100,000us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod
Interest rate at December 31 3.77%us-gaap_DebtInstrumentInterestRateEffectivePercentage 2.98%us-gaap_DebtInstrumentInterestRateEffectivePercentage 4.40%us-gaap_DebtInstrumentInterestRateEffectivePercentage