Investment Securities - Contractual Maturities of Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year	$ 13,524
Due after one year through five years	223,219
Due after five years through ten years	321,954
Due after ten years	286,332
Residential mortgage-backed securities	1,185,165
Amortized Cost	2,030,194	$ 1,842,691
Fair Value
Due in one year	13,580
Due after one year through five years	226,330
Due after five years through ten years	329,369
Due after ten years	273,053
Residential mortgage-backed securities	1,146,450
Total investment securities held to maturity	$ 1,988,782	$ 1,837,620