Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available for Sale Amortized Cost
Within one year	$ 0	$ 0
After one year but within five years	19,714	20,051
After five years but within ten years	170,118	179,812
After ten years	1,034,395	647,928
Amortized Cost	1,224,227	847,791
Available for Sale Fair Value
Within one year	0	0
After one year but within five years	19,423	19,825
After five years but within ten years	163,731	175,583
After ten years	1,021,925	642,091
Available-for-sale securities, fair value total	1,205,079	837,499
Held to Maturity, Amortized Cost
Within one year	1,018	322
After one year but within five years	6,703	5,895
After five years but within ten years	29,613	18,751
After ten years	112,616	136,218
Amortized Cost	149,950	161,186
Held to Maturity, Fair Value
Within one year	1,016	320
After one year but within five years	6,701	5,894
After five years but within ten years	29,547	18,873
After ten years	112,167	137,766
Held-to-maturity securities, fair value total	$ 149,431	162,853
Mutual Fund
Available for Sale Fair Value
Available-for-sale securities, fair value total		$ 600