Fair Value Measurements - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Fair Value Measurements Disclosure [Line Items]
Securities held to maturity, fair value	$ 639,631us-gaap_HeldToMaturitySecurities	$ 536,583us-gaap_HeldToMaturitySecurities
Fair Value, Measurements, Nonrecurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Recognized impairment	300
Securities held to maturity, fair value	$ 400

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Name:	us-gaap_HeldToMaturitySecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant