Acquisition Activity - Additional Information (Details) $ in Thousands		3 Months Ended		9 Months Ended
	Jul. 31, 2017 USD ($) bank shares	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Business Acquisition [Line Items]
Increase in goodwill due to adjustments				$ 431,827
Merger related costs		$ 28,500		29,600
Sabadell United
Business Acquisition [Line Items]
Loans acquired		4,026,240		4,026,240
Interest-bearing deposits acquired		4,383,049		4,383,049
Number of offices acquired | bank	25
Cash consideration	$ 809,159
Percentage of voting interests acquired	100.00%
Increase in goodwill due to adjustments		431,800
Sabadell United | Fair Value Adjustments
Business Acquisition [Line Items]
amortization of premiums and accretion of discounts on acquired loans, securities, and deposits		3,000	$ 9,700	20,300	$ 29,100
Elimination of amortization of premiums and accretion of discounts on previously acquired loans, securities, FDIC indemnification asset, and deposits		20	1,000	1,300	3,100
Allowance for loan and lease losses acquired		900	1,500	6,400	4,400
Amortization of intangible assets acquired		$ 1,100	$ 3,400	$ 7,700	$ 10,300
Sabadell United | Common Stock
Business Acquisition [Line Items]
Common stock issued (shares) | shares	2,610,304