Securities - Amortized Cost and Estimated Fair Value of Securities - Securities Held-to-maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Securities held-to-maturity:
Amortized Cost	$ 379,550	$ 324,010
Gross Unrealized Gains	1,313	536
Gross Unrealized Losses	(3,018)	(5,798)
Estimated Fair Value	377,845	318,748
Agency mortgage-backed securities
Securities held-to-maturity:
Amortized Cost	290,418	233,409
Gross Unrealized Gains	259	15
Gross Unrealized Losses	(2,341)	(3,554)
Estimated Fair Value	288,336	229,870
Municipals - exempt from Federal tax
Securities held-to-maturity:
Amortized Cost	89,132	90,601
Gross Unrealized Gains	1,054	521
Gross Unrealized Losses	(677)	(2,244)
Estimated Fair Value	$ 89,509	$ 88,878