Investment Securities - Amortized cost and fair value of available-for-sale investment securities portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2016		Dec. 31, 2015
Available-for-sale securities:
Amortized cost	$ 682,637		$ 691,533
Gross unrealized gains	14,990		5,939
Gross unrealized losses	(1,370)		(5,909)
Estimated fair value	696,257		691,563
Obligations of U.S. Government agencies
Available-for-sale securities:
Amortized cost	102,797		147,135
Gross unrealized gains	452		274
Gross unrealized losses	(207)		(1,433)
Estimated fair value	103,042		145,976
Obligations of states and political subdivisions
Available-for-sale securities:
Amortized cost	127,699		136,499
Gross unrealized gains	6,366		3,827
Gross unrealized losses	(17)		(266)
Estimated fair value	134,048		140,060
U.S. GSE's MBS-residential
Available-for-sale securities:
Amortized cost	127,509	[1]	121,646
Gross unrealized gains	2,521	[1]	752
Gross unrealized losses	(21)	[1]	(890)
Estimated fair value	130,009	[1]	121,508
U.S. GSE's MBS - commercial
Available-for-sale securities:
Amortized cost	28,855	[1]	21,805
Gross unrealized gains	527	[1]	125
Gross unrealized losses		[1]	(261)
Estimated fair value	29,382	[1]	21,669
U.S. GSE's CMO
Available-for-sale securities:
Amortized cost	177,314		146,782
Gross unrealized gains	3,353		437
Gross unrealized losses	(107)		(1,642)
Estimated fair value	180,560		145,577
Corporate bonds
Available-for-sale securities:
Amortized cost	118,463		117,666
Gross unrealized gains	1,771		524
Gross unrealized losses	(1,018)		(1,417)
Estimated fair value	$ 119,216		$ 116,773

[1] Government sponsored entities