Securities - Amortized Cost and Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due within one year	$ 41,421
Due after one year through five years	48,858
Due after five years through ten years	81,381
Due after ten years	61,925
Amortized Cost	1,201,389	$ 674,104
Fair Value
Due within one year	41,815
Due after one year through five years	49,351
Due after five years through ten years	81,580
Due after ten years	60,153
Fair Value	1,177,606	671,488
Other debt securities
Amortized Cost
Debt securities, without fixed maturity, amortized cost	26,500
Amortized Cost	35,308	17,106
Fair Value
Debt securities, without fixed maturity, fair value	26,195
Fair Value	34,965	17,290
Government agency mortgage backed securities \| Residential mortgage backed securities
Amortized Cost
Debt securities, without fixed maturity, amortized cost	573,236
Amortized Cost	573,236	196,121
Fair Value
Debt securities, without fixed maturity, fair value	560,587
Fair Value	560,587	193,950
Government agency mortgage backed securities \| Commercial mortgage-backed securities
Amortized Cost
Debt securities, without fixed maturity, amortized cost	22,094
Amortized Cost	22,094	31,015
Fair Value
Debt securities, without fixed maturity, fair value	21,735
Fair Value	21,735	31,170
Government agency collateralized mortgage obligations \| Residential mortgage backed securities
Amortized Cost
Debt securities, without fixed maturity, amortized cost	316,642
Amortized Cost	316,642	180,258
Fair Value
Debt securities, without fixed maturity, fair value	307,228
Fair Value	307,228	176,639
Government agency collateralized mortgage obligations \| Commercial mortgage-backed securities
Amortized Cost
Debt securities, without fixed maturity, amortized cost	29,332
Amortized Cost	29,332	5,019
Fair Value
Debt securities, without fixed maturity, fair value	28,962
Fair Value	$ 28,962	$ 5,006

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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