Investment Securities - Contractual Maturities of Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year	$ 18,801
Due after one year through five years	127,790
Due after five years through ten years	80,582
Due after ten years	136,739
Securities without a single maturity date	1,525,041
Amortized Cost	1,888,953	$ 1,511,034
Fair Value
Due in one year	18,876
Due after one year through five years	124,100
Due after five years through ten years	79,923
Due after ten years	132,723
Securities without a single maturity date	1,477,845
Total investment securities available for sale, Fair Value	$ 1,833,467	$ 1,493,905