Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment securities available-for-sale:
Total	$ 1,214,203	$ 1,111,168
Unrealized Gain	44,030	6,263
Unrealized Loss	(1,578)	(14,209)
Estimated Fair Value	1,256,655	1,103,222
Investment securities held-to-maturity:
Amortized Cost	40,433	45,210
Unrealized Gain	967	148
Unrealized Loss	(98)	(686)
Estimated Fair Value	41,302	44,672
Total investment securities
Amortized Cost	1,254,636	1,156,378
Unrealized Gain	44,997	6,411
Unrealized Loss	(1,676)	(14,895)
Estimated Fair Value	1,297,957	1,147,894
U.S. Treasury
Investment securities available-for-sale:
Total	60,457	59,688
Unrealized Gain	3,733	1,224
Unrealized Loss	0	0
Estimated Fair Value	64,190	60,912
Agency
Investment securities available-for-sale:
Total	251,329	128,958
Unrealized Gain	10,778	1,631
Unrealized Loss	(549)	(519)
Estimated Fair Value	261,558	130,070
Corporate
Investment securities available-for-sale:
Total	125,564	104,158
Unrealized Gain	2,255	291
Unrealized Loss	(17)	(906)
Estimated Fair Value	127,802	103,543
Municipal bonds
Investment securities available-for-sale:
Total	279,593	238,914
Unrealized Gain	14,562	1,941
Unrealized Loss	(56)	(2,225)
Estimated Fair Value	294,099	238,630
Collateralized mortgage obligation: residential
Investment securities available-for-sale:
Total	10,514	24,699
Unrealized Gain	132	64
Unrealized Loss	(4)	(425)
Estimated Fair Value	10,642	24,338
Mortgage-backed securities: residential
Investment securities available-for-sale:
Total	486,746	554,751
Unrealized Gain	12,570	1,112
Unrealized Loss	(952)	(10,134)
Estimated Fair Value	498,364	545,729
Investment securities held-to-maturity:
Amortized Cost	38,686	43,381
Unrealized Gain	967	148
Unrealized Loss	(98)	(686)
Estimated Fair Value	39,555	42,843
Other
Investment securities held-to-maturity:
Amortized Cost	1,747	1,829
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated Fair Value	$ 1,747	$ 1,829