Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	$ 3,475,507	$ 3,213,655
Available-for-sale Securities, Gross Unrealized Gain	69,709	32,386
Available-for-sale Securities, Gross Unrealized Loss	(12,986)	(22,211)
Securities available‑for‑sale	3,532,230	3,223,830
Government agency and government-sponsored enterprise pass through securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	375,357	499,185
Available-for-sale Securities, Gross Unrealized Gain	4,569	6,222
Available-for-sale Securities, Gross Unrealized Loss	(2,385)	(2,964)
Securities available‑for‑sale	377,541	502,443
Government agency and government-sponsored enterprise collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	175,429	145,258
Available-for-sale Securities, Gross Unrealized Gain	1,655	1,528
Available-for-sale Securities, Gross Unrealized Loss	(1,134)	(497)
Securities available‑for‑sale	175,950	146,289
Private label collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	131,149	122,707
Available-for-sale Securities, Gross Unrealized Gain	4,001	4,199
Available-for-sale Securities, Gross Unrealized Loss	(600)	(1,437)
Securities available‑for‑sale	134,550	125,469
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	1,501,196	1,447,064
Available-for-sale Securities, Gross Unrealized Gain	49,362	15,406
Available-for-sale Securities, Gross Unrealized Loss	(1,408)	(6,011)
Securities available‑for‑sale	1,549,150	1,456,459
Government agency and government-sponsored enterprise commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	996,963	555,552
Available-for-sale Securities, Gross Unrealized Gain	5,764	1,798
Available-for-sale Securities, Gross Unrealized Loss	(6,813)	(9,658)
Securities available‑for‑sale	995,914	547,692
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	17,000	47,100
Available-for-sale Securities, Gross Unrealized Gain	2,125	680
Available-for-sale Securities, Gross Unrealized Loss	0	(271)
Securities available‑for‑sale	19,125	47,509
Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	19,119	155,440
Available-for-sale Securities, Gross Unrealized Gain	66	1,685
Available-for-sale Securities, Gross Unrealized Loss	(9)	(238)
Securities available‑for‑sale	19,176	156,887
SBA asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	166,949	179,085
Available-for-sale Securities, Gross Unrealized Gain	1,363	510
Available-for-sale Securities, Gross Unrealized Loss	(284)	(750)
Securities available‑for‑sale	168,028	178,845
Asset-backed and other securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	92,345	62,264
Available-for-sale Securities, Gross Unrealized Gain	804	358
Available-for-sale Securities, Gross Unrealized Loss	(353)	(385)
Securities available‑for‑sale	$ 92,796	$ 62,237