Investment Securities - Summary of Amortized Cost, Gross Unrealized Gains and Losses and Approximate Fair Values of 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, Amortized Cost	$ 2,187,402	$ 2,017,881
Available for sale securities, Unrealized Gains	17,329	22,407
Available for sale securities, Unrealized Losses	24,161	9,592
Available for sale securities, Fair Value	2,180,570	2,030,696
U.S. Government agencies and sponsored enterprises obligations
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	91,364	43,471
Available for sale securities, Unrealized Gains	30	38
Available for sale securities, Unrealized Losses	1,384	671
Available for sale securities, Fair Value	90,010	42,838
U.S. Government agencies and sponsored enterprises mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	611,868	600,310
Available for sale securities, Unrealized Gains	187	1,716
Available for sale securities, Unrealized Losses	15,619	6,789
Available for sale securities, Fair Value	596,436	595,237
State and municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	26,260	26,766
Available for sale securities, Unrealized Gains	99	125
Available for sale securities, Unrealized Losses	1,323	719
Available for sale securities, Fair Value	25,036	26,172
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	670,439	608,340
Available for sale securities, Unrealized Gains	1,388	2,306
Available for sale securities, Unrealized Losses	218	100
Available for sale securities, Fair Value	671,609	610,546
Corporate bonds and other debt securities
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	787,471	738,994
Available for sale securities, Unrealized Gains	15,625	18,222
Available for sale securities, Unrealized Losses	5,617	1,313
Available for sale securities, Fair Value	$ 797,479	$ 755,903

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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