Securities - Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Due within one year	$ 39,310
Due after one year through five years	44,304
Due after five years through ten years	81,825
Due after ten years	59,051
Amortized Cost	1,264,529	$ 674,104
Available-for-sale Securities, Fair Value [Abstract]
Due within one year	39,649
Due after one year through five years	44,788
Due after five years through ten years	82,781
Due after ten years	58,001
Fair Value	1,250,777	671,488
Residential mortgage backed securities \| Government agency mortgage backed securities
Cash and Cash Equivalents [Line Items]
Without single maturity date, amortized cost	621,690
Amortized Cost	621,690	196,121
Available-for-sale Securities, Fair Value [Abstract]
Without single maturity date, fair value	613,283
Fair Value	613,283	193,950
Residential mortgage backed securities \| Government agency collateralized mortgage obligations
Cash and Cash Equivalents [Line Items]
Without single maturity date, amortized cost	332,697
Amortized Cost	332,697	180,258
Available-for-sale Securities, Fair Value [Abstract]
Without single maturity date, fair value	326,989
Fair Value	326,989	176,639
Commercial mortgage backed securities \| Government agency mortgage backed securities
Cash and Cash Equivalents [Line Items]
Without single maturity date, amortized cost	21,957
Amortized Cost	21,957	31,015
Available-for-sale Securities, Fair Value [Abstract]
Without single maturity date, fair value	21,830
Fair Value	21,830	31,170
Commercial mortgage backed securities \| Government agency collateralized mortgage obligations
Cash and Cash Equivalents [Line Items]
Without single maturity date, amortized cost	28,446
Amortized Cost	28,446	5,019
Available-for-sale Securities, Fair Value [Abstract]
Without single maturity date, fair value	28,335
Fair Value	28,335	5,006
Other debt securities
Cash and Cash Equivalents [Line Items]
Without single maturity date, amortized cost	35,249
Amortized Cost	44,046	17,106
Available-for-sale Securities, Fair Value [Abstract]
Without single maturity date, fair value	35,121
Fair Value	$ 43,927	$ 17,290

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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