Investment Securities, Available-for-sale Securities, Gross Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	$ 118,414	$ 77,814
12 months or more - Fair Value	400,964	481,405
Total - Fair Value	519,378	559,219
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	2,451	637
12 months or more - Gross Unrealized Loss	15,190	8,224
Total - Gross Unrealized Loss	17,641	8,861
U. S. Government Sponsored Enterprises [Member]
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	68,671	29,734
12 months or more - Fair Value	82,034	98,090
Total - Fair Value	150,705	127,824
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	1,302	266
12 months or more - Gross Unrealized Loss	2,857	1,800
Total - Gross Unrealized Loss	4,159	2,066
Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	34,750	48,080
12 months or more - Fair Value	247,070	276,094
Total - Fair Value	281,820	324,174
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	1,061	371
12 months or more - Gross Unrealized Loss	10,147	4,454
Total - Gross Unrealized Loss	11,208	4,825
Corporate Bonds [Member]
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	14,993	0
12 months or more - Fair Value	9,984	40,162
Total - Fair Value	24,977	40,162
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	88	0
12 months or more - Gross Unrealized Loss	67	108
Total - Gross Unrealized Loss	155	108
Small Business Administration-Guaranteed Participation Securities [Member]
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	0	0
12 months or more - Fair Value	61,876	67,059
Total - Fair Value	61,876	67,059
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	0	0
12 months or more - Gross Unrealized Loss	2,119	1,862
Total - Gross Unrealized Loss	$ 2,119	$ 1,862