Investment Securities - Continuous Unrealized Loss Position of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Securities available for sale:
Less than 12 months, Fair Value	$ 648,692	$ 548,803
Less than 12 months, Gross unrealized (losses)	(16,184)	(8,454)
12 months or longer, Fair value	188,936	58,068
12 months or longer, Gross unrealized (losses)	(11,304)	(2,378)
Total, Fair value	837,628	606,871
Total, Gross unrealized (losses)	(27,488)	(10,832)
U.S. government and agency obligations
Securities available for sale:
Less than 12 months, Fair Value	48,812	29,654
Less than 12 months, Gross unrealized (losses)	(1,012)	(282)
12 months or longer, Fair value	4,750	0
12 months or longer, Gross unrealized (losses)	(231)	0
Total, Fair value	53,562	29,654
Total, Gross unrealized (losses)	(1,243)	(282)
Mortgage-backed securities and collateralized mortgage obligations
Securities available for sale:
Less than 12 months, Fair Value	580,116	514,283
Less than 12 months, Gross unrealized (losses)	(14,943)	(8,037)
12 months or longer, Fair value	170,350	48,788
12 months or longer, Gross unrealized (losses)	(9,909)	(1,658)
Total, Fair value	750,466	563,071
Total, Gross unrealized (losses)	(24,852)	(9,695)
Corporate debt securities
Securities available for sale:
Less than 12 months, Fair Value	19,764	4,866
Less than 12 months, Gross unrealized (losses)	(229)	(135)
12 months or longer, Fair value	9,206	4,624
12 months or longer, Gross unrealized (losses)	(794)	(376)
Total, Fair value	28,970	9,490
Total, Gross unrealized (losses)	(1,023)	(511)
Trust preferred securities
Securities available for sale:
Less than 12 months, Fair Value	0	0
Less than 12 months, Gross unrealized (losses)	0	0
12 months or longer, Fair value	4,630	4,656
12 months or longer, Gross unrealized (losses)	(370)	(344)
Total, Fair value	4,630	4,656
Total, Gross unrealized (losses)	$ (370)	$ (344)