Investment Securities - Continuous Unrealized Loss Position of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Securities available for sale:
Less than 12 months, Fair Value	$ 501,886	$ 548,803
Less than 12 months, Gross unrealized (losses)	(10,682)	(8,454)
12 months or longer, Fair value	407,361	58,068
12 months or longer, Gross unrealized (losses)	(22,695)	(2,378)
Total, Fair value	909,247	606,871
Total, Gross unrealized (losses)	(33,377)	(10,832)
U.S. government and agency obligations
Securities available for sale:
Less than 12 months, Fair Value	38,696	29,654
Less than 12 months, Gross unrealized (losses)	(1,169)	(282)
12 months or longer, Fair value	14,467	0
12 months or longer, Gross unrealized (losses)	(481)	0
Total, Fair value	53,163	29,654
Total, Gross unrealized (losses)	(1,650)	(282)
Mortgage-backed securities and collateralized mortgage obligations
Securities available for sale:
Less than 12 months, Fair Value	419,490	514,283
Less than 12 months, Gross unrealized (losses)	(8,722)	(8,037)
12 months or longer, Fair value	383,623	48,788
12 months or longer, Gross unrealized (losses)	(21,485)	(1,658)
Total, Fair value	803,113	563,071
Total, Gross unrealized (losses)	(30,207)	(9,695)
Corporate debt securities
Securities available for sale:
Less than 12 months, Fair Value	39,107	4,866
Less than 12 months, Gross unrealized (losses)	(384)	(135)
12 months or longer, Fair value	9,271	4,624
12 months or longer, Gross unrealized (losses)	(729)	(376)
Total, Fair value	48,378	9,490
Total, Gross unrealized (losses)	(1,113)	(511)
Trust preferred securities
Securities available for sale:
Less than 12 months, Fair Value	4,593	0
Less than 12 months, Gross unrealized (losses)	(407)	0
12 months or longer, Fair value	0	4,656
12 months or longer, Gross unrealized (losses)	0	(344)
Total, Fair value	4,593	4,656
Total, Gross unrealized (losses)	$ (407)	$ (344)