Loans and Allowance for Loan Losses, Based on Impairment Review Method (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	$ 0		$ 0
Collectively evaluated for impairment	45,571		46,327
Total ending allowance balance	45,571	$ 45,944	46,327	$ 46,935	$ 47,035	$ 47,714
Loans [Abstract]
Individually evaluated for impairment	26,621		26,535
Collectively evaluated for impairment	3,216,327		3,131,797
Total loans	3,242,948		3,158,332
Commercial [Member]
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	4,022		4,071
Total ending allowance balance	4,022	4,024	4,071	4,073	3,840	4,019
Loans [Abstract]
Individually evaluated for impairment	3,551		4,129
Collectively evaluated for impairment	205,848		219,253
Total loans	209,399		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	41,087		42,088
Total ending allowance balance	41,087	41,529	42,088	42,752	43,091	43,597
Loans [Abstract]
Individually evaluated for impairment	23,070		22,406
Collectively evaluated for impairment	3,001,805		2,904,950
Total loans	3,024,875		2,927,356
Installment [Member]
Ending allowance balance attributable to loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	462		168
Total ending allowance balance	462	$ 391	168	$ 110	$ 104	$ 98
Loans [Abstract]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	8,674		7,594
Total loans	$ 8,674		$ 7,594