Investment Securities - Summary of Estimated Fair Values and Gross Unrealized Losses on Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities at Fair Value, Less than 12 months	$ 924,631	$ 462,039
Available for sale securities Unrealized Loss, Less than 12 Months	14,107	3,446
Available for sale securities at Fair Value, 12 months or more	206,540	217,622
Available for sale securities Unrealized Loss, 12 months or more	10,054	6,146
Available for sale securities, Total Fair Value	1,131,171	679,661
Available for sale securities, Total Unrealized Loss	24,161	9,592
U.S. Government agencies and sponsored enterprises obligations
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities at Fair Value, Less than 12 months	54,803	31,518
Available for sale securities Unrealized Loss, Less than 12 Months	945	268
Available for sale securities at Fair Value, 12 months or more	6,104	7,157
Available for sale securities Unrealized Loss, 12 months or more	439	403
Available for sale securities, Total Fair Value	60,907	38,675
Available for sale securities, Total Unrealized Loss	1,384	671
U.S. Government agencies and sponsored enterprises mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities at Fair Value, Less than 12 months	396,538	207,735
Available for sale securities Unrealized Loss, Less than 12 Months	7,581	1,836
Available for sale securities at Fair Value, 12 months or more	167,087	175,810
Available for sale securities Unrealized Loss, 12 months or more	8,038	4,953
Available for sale securities, Total Fair Value	563,625	383,545
Available for sale securities, Total Unrealized Loss	15,619	6,789
State and municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities at Fair Value, Less than 12 months	186	192
Available for sale securities Unrealized Loss, Less than 12 Months	7	2
Available for sale securities at Fair Value, 12 months or more	22,707	23,813
Available for sale securities Unrealized Loss, 12 months or more	1,316	717
Available for sale securities, Total Fair Value	22,893	24,005
Available for sale securities, Total Unrealized Loss	1,323	719
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities at Fair Value, Less than 12 months	115,856	36,542
Available for sale securities Unrealized Loss, Less than 12 Months	218	100
Available for sale securities at Fair Value, 12 months or more	0	0
Available for sale securities Unrealized Loss, 12 months or more	0	0
Available for sale securities, Total Fair Value	115,856	36,542
Available for sale securities, Total Unrealized Loss	218	100
Corporate bonds and other debt securities
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities at Fair Value, Less than 12 months	357,248	186,052
Available for sale securities Unrealized Loss, Less than 12 Months	5,356	1,240
Available for sale securities at Fair Value, 12 months or more	10,642	10,842
Available for sale securities Unrealized Loss, 12 months or more	261	73
Available for sale securities, Total Fair Value	367,890	196,894
Available for sale securities, Total Unrealized Loss	$ 5,617	$ 1,313

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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-Publisher FASB
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