Loans and Allowance for Loan Losses, Based on Impairment Review Method (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	$ 0		$ 0
Collectively evaluated for impairment	45,149		46,327
Total ending allowance balance	45,149	$ 45,571	46,327	$ 46,512	$ 46,935	$ 47,714
Loans [Abstract]
Individually evaluated for impairment	27,120		26,535
Collectively evaluated for impairment	3,254,885		3,131,797
Total loans	3,282,005		3,158,332
Commercial [Member]
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	4,056		4,071
Total ending allowance balance	4,056	4,022	4,071	4,045	4,073	4,019
Loans [Abstract]
Individually evaluated for impairment	3,984		4,129
Collectively evaluated for impairment	204,810		219,253
Total loans	208,794		223,382
1 to 4 Family Residential Real Estate [Member]
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	40,647		42,088
Total ending allowance balance	40,647	41,087	42,088	42,340	42,752	43,597
Loans [Abstract]
Individually evaluated for impairment	23,136		22,406
Collectively evaluated for impairment	3,041,145		2,904,950
Total loans	3,064,281		2,927,356
Installment [Member]
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	446		168
Total ending allowance balance	446	$ 462	168	$ 127	$ 110	$ 98
Loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	8,930		7,594
Total loans	$ 8,930		$ 7,594