Shareholders' Equity, Capital Ratios and Other Regulatory Matters - Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands						9 Months Ended
	Jul. 31, 2017	Mar. 07, 2017	Dec. 07, 2016	May 09, 2016	Aug. 05, 2015	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Class of Stock [Line Items]
Liquidation preference value						$ 137,500
Carrying Amount						$ 132,097		$ 132,097
Number of shares authorized to be repurchased							950,000
Shares authorized to be repurchased						747,494
Proceeds from common stock issued						$ 485,751	$ 0
Series B Preferred Stock
Class of Stock [Line Items]
Ownership per share					6.625%
Liquidation preference value					$ 80,000
Carrying Amount						76,812		76,812
Series C Preferred Stock
Class of Stock [Line Items]
Ownership per share				6.60%
Liquidation preference value				$ 57,500
Carrying Amount						$ 55,285		$ 55,285
Common Stock
Class of Stock [Line Items]
Common stock issued (in shares)		6,100,000	3,593,750
Price per share of stock of issued		$ 83.00	$ 81.50
Proceeds from common stock issued		$ 485,200	$ 279,200
Sabadell United
Class of Stock [Line Items]
Cash consideration	$ 809,159
Sabadell United | Common Stock
Class of Stock [Line Items]
Common stock issued (shares)	2,610,304
Common stock issued	$ 211,043
Common stock issued (in dollars per share)	$ 80.85