Loans and the Allowance for Credit Losses - Summary of Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, With no specific allowance recorded	$ 61,759	$ 45,474
Total impaired loans, Unpaid principal balance	69,993	55,296
Recorded Investment, With no specific allowance recorded	50,470	41,062
Total impaired loans, Recorded investment	58,704	50,884
Unpaid Principal Balance, With a specific allowance recorded	8,234	9,822
Recorded Investment, With a specific allowance recorded	8,234	9,822
Related Allowance, With a specific allowance recorded	3,701	3,798
Average recorded investment, with no related specific allowance	46,123	48,144
Interest income recognized, With no related specific allowance	522	283
Average recorded investment, With a specific allowance recorded	9,028	3,058
Interest income recognized, With a specific allowance recorded	19	13
Total impaired loans, Average recorded investment	55,151	51,202
Total impaired loans, Interest income recognized	541	296
Commercial Real Estate - Land and Construction [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, With no specific allowance recorded	3,101	1,588
Recorded Investment, With no specific allowance recorded	2,476	1,488
Average recorded investment, with no related specific allowance	2,128	2,450
Interest income recognized, With no related specific allowance	16	2
Commercial Real Estate - Improved Property [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, With no specific allowance recorded	30,138	16,480
Recorded Investment, With no specific allowance recorded	22,481	14,684
Unpaid Principal Balance, With a specific allowance recorded	6,466	7,980
Recorded Investment, With a specific allowance recorded	6,466	7,980
Related Allowance, With a specific allowance recorded	2,828	2,765
Average recorded investment, with no related specific allowance	18,932	19,158
Interest income recognized, With no related specific allowance	223	20
Average recorded investment, With a specific allowance recorded	7,223	729
Interest income recognized, With a specific allowance recorded		1
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, With no specific allowance recorded	3,094	3,152
Recorded Investment, With no specific allowance recorded	2,432	2,597
Unpaid Principal Balance, With a specific allowance recorded	1,768	1,842
Recorded Investment, With a specific allowance recorded	1,768	1,842
Related Allowance, With a specific allowance recorded	873	1,033
Average recorded investment, with no related specific allowance	2,513	3,532
Interest income recognized, With no related specific allowance	13	32
Average recorded investment, With a specific allowance recorded	1,805	2,329
Interest income recognized, With a specific allowance recorded	19	12
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, With no specific allowance recorded	20,943	20,077
Recorded Investment, With no specific allowance recorded	19,132	18,544
Average recorded investment, with no related specific allowance	18,715	19,463
Interest income recognized, With no related specific allowance	230	182
Home Equity [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, With no specific allowance recorded	2,824	2,890
Recorded Investment, With no specific allowance recorded	2,617	2,663
Average recorded investment, with no related specific allowance	2,641	2,367
Interest income recognized, With no related specific allowance	20	19
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, With no specific allowance recorded	1,659	1,287
Recorded Investment, With no specific allowance recorded	1,332	1,086
Average recorded investment, with no related specific allowance	1,194	1,174
Interest income recognized, With no related specific allowance	$ 20	$ 28

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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524

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

-Publisher FASB

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-Topic 310

-SubTopic 10

-Section 50

-Paragraph 14A

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-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524

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