Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Securities Available -for- Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized Cost	$ 1,759,711	$ 1,221,511
Securities available-for-sale, Gross Unrealized Gains	10,417	1,658
Securities available-for-sale, Gross Unrealized Losses	15,289	35,917
Securities available-for-sale, Estimated Fair Value	1,754,839	1,187,252
U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized Cost	100,372	100,413
Securities available-for-sale, Gross Unrealized Losses	2,806	3,628
Securities available-for-sale, Estimated Fair Value	97,566	96,785
Obligations of U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized Cost	118,498	60,975
Securities available-for-sale, Gross Unrealized Gains	596	316
Securities available-for-sale, Gross Unrealized Losses	227	284
Securities available-for-sale, Estimated Fair Value	118,867	61,007
Residential Pass-through \| Guaranteed by GNMA
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized Cost	92,698	85,052
Securities available-for-sale, Gross Unrealized Gains	497	146
Securities available-for-sale, Gross Unrealized Losses	1,143	2,093
Securities available-for-sale, Estimated Fair Value	92,052	83,105
Residential Pass-through \| Issued by FNMA and FHLMC
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized Cost	764,369	594,874
Securities available-for-sale, Gross Unrealized Gains	3,999	694
Securities available-for-sale, Gross Unrealized Losses	4,659	10,367
Securities available-for-sale, Estimated Fair Value	763,709	585,201
Other Residential Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized Cost	319,706	36,339
Securities available-for-sale, Gross Unrealized Gains	2,275	8
Securities available-for-sale, Gross Unrealized Losses	752	1,178
Securities available-for-sale, Estimated Fair Value	321,229	35,169
Commercial Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized Cost	134,345	114,383
Securities available-for-sale, Gross Unrealized Gains	1,101	287
Securities available-for-sale, Gross Unrealized Losses	3,134	5,255
Securities available-for-sale, Estimated Fair Value	132,312	109,415
Total MBS
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized Cost	1,311,118	830,648
Securities available-for-sale, Gross Unrealized Gains	7,872	1,135
Securities available-for-sale, Gross Unrealized Losses	9,688	18,893
Securities available-for-sale, Estimated Fair Value	1,309,302	812,890
Obligations of States and Municipal Subdivisions
Schedule Of Available For Sale Securities [Line Items]
Securities available-for-sale, Amortized Cost	229,723	229,475
Securities available-for-sale, Gross Unrealized Gains	1,949	207
Securities available-for-sale, Gross Unrealized Losses	2,568	13,112
Securities available-for-sale, Estimated Fair Value	$ 229,104	$ 216,570

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
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-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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 320
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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
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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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