Investment Securities - Continuous Unrealized Loss Position of Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment securities held-to-maturity:
Less than 12 months, Fair Value	$ 91,928	$ 204,393
Less than 12 months, Gross unrealized (losses)	(3,197)	(2,978)
12 months or longer, Fair value	148,991	30,046
12 months or longer, Gross unrealized (losses)	(9,914)	(515)
Total, Fair value	240,919	234,439
Total, Gross unrealized (losses)	(13,111)	(3,493)
U.S. government and agency obligations
Investment securities held-to-maturity:
Less than 12 months, Fair Value	19,634	8,344
Less than 12 months, Gross unrealized (losses)	(362)	(58)
12 months or longer, Fair value	3,223	0
12 months or longer, Gross unrealized (losses)	(185)	0
Total, Fair value	22,857	8,344
Total, Gross unrealized (losses)	(547)	(58)
Mortgage-backed securities and collateralized mortgage obligations
Investment securities held-to-maturity:
Less than 12 months, Fair Value	72,294	196,049
Less than 12 months, Gross unrealized (losses)	(2,835)	(2,920)
12 months or longer, Fair value	145,768	30,046
12 months or longer, Gross unrealized (losses)	(9,729)	(515)
Total, Fair value	218,062	226,095
Total, Gross unrealized (losses)	$ (12,564)	$ (3,435)