Debt Securities Available for Sale - Continuous Unrealized Loss Position of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Securities available for sale:
Less than 12 months, Fair Value	$ 44,845	$ 217,762
Less than 12 months, Gross unrealized (losses)	(379)	(1,748)
12 months or longer, Fair value	613,076	552,442
12 months or longer, Gross unrealized (losses)	(11,309)	(18,310)
Total, Fair value	657,921	770,204
Total, Gross unrealized (losses)	(11,688)	(20,058)
U.S. government and agency obligations
Securities available for sale:
Less than 12 months, Fair Value	0	14,668
Less than 12 months, Gross unrealized (losses)	0	(202)
12 months or longer, Fair value	34,351	29,437
12 months or longer, Gross unrealized (losses)	(344)	(515)
Total, Fair value	34,351	44,105
Total, Gross unrealized (losses)	(344)	(717)
Mortgage-backed securities and collateralized mortgage obligations
Securities available for sale:
Less than 12 months, Fair Value	23,038	176,614
Less than 12 months, Gross unrealized (losses)	(187)	(1,034)
12 months or longer, Fair value	559,793	509,397
12 months or longer, Gross unrealized (losses)	(9,902)	(16,402)
Total, Fair value	582,831	686,011
Total, Gross unrealized (losses)	(10,089)	(17,436)
Corporate debt securities
Securities available for sale:
Less than 12 months, Fair Value	21,807	26,480
Less than 12 months, Gross unrealized (losses)	(192)	(512)
12 months or longer, Fair value	14,395	9,358
12 months or longer, Gross unrealized (losses)	(600)	(643)
Total, Fair value	36,202	35,838
Total, Gross unrealized (losses)	(792)	(1,155)
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	4,537	4,250
12 months or longer, Gross unrealized (losses)	(463)	(750)
Total, Fair value	4,537	4,250
Total, Gross unrealized (losses)	$ (463)	$ (750)