Parent Company Information - Schedule of Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net Income		$ 26,098	$ 39,649	$ 50,065	$ 46,095	$ 50,090	$ 42,842	$ 39,027	$ 36,187	$ 161,907	$ 168,146	$ 102,957
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation										12,204	10,032	7,575
Net amortization of premiums and accretion of discounts on securities										46,346	24,310	22,080
Losses (gains) on securities transactions, net										20	(777)	(2,487)
Net change in:
Other assets										(57,353)	47,458	(71,263)
Net cash provided by operating activities										619,230	419,177	154,929
Cash flows from investing activities:
Sales										2,727	4,782	140,640
Cash and cash equivalents acquired in acquisitions										0	0	201,025
Net cash used in investing activities										(1,547,548)	(1,527,313)	(1,700,399)
Cash flows from financing activities:
Proceeds from issuance of long-term borrowings, net										1,065,000	385,000	162,792
Proceeds from issuance of preferred stock, net										98,101	0	111,590
Dividends paid to preferred shareholders										(6,277)	(7,188)	(3,813)
Dividends paid to common shareholders										(115,881)	(111,813)	(102,279)
Purchase of common shares to treasury										(2,645)	(3,191)	(2,108)
Common stock issued, net	$ 106,400									8,207	112,085	7,898
Net cash provided by financing activities										951,927	1,086,837	1,128,863
Net change in cash and cash equivalents										23,609	(21,299)	(416,607)
Cash and cash equivalents at beginning of year					392,501				413,800	392,501	413,800	830,407
Cash and cash equivalents at end of year	392,501	416,110				392,501				416,110	392,501	413,800
Parent Company
Cash flows from operating activities:
Net Income										161,907	168,146	102,957
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed (earnings) losses of subsidiary										(44,664)	(83,578)	10,978
Stock-based compensation										12,204	10,032	7,575
Net amortization of premiums and accretion of discounts on securities										197	163	162
Losses (gains) on securities transactions, net										0	(239)	0
Net change in:
Other assets										(89)	8,007	(41,452)
Accrued expenses and other liabilities										8,737	18,381	9,604
Net cash provided by operating activities										138,292	120,912	89,824
Cash flows from investing activities:
Sales										0	739	49
Cash and cash equivalents acquired in acquisitions										0	0	109
Capital contributions to subsidiary										(98,000)	(106,000)	(115,000)
Subordinated debt issued by subsidiary										0	0	(100,000)
Net cash used in investing activities										(98,000)	(105,261)	(214,842)
Cash flows from financing activities:
Proceeds from issuance of long-term borrowings, net										0	0	98,897
Proceeds from issuance of preferred stock, net										98,101	0	111,590
Dividends paid to preferred shareholders										(6,277)	(7,188)	(3,813)
Dividends paid to common shareholders										(115,881)	(111,813)	(102,279)
Purchase of common shares to treasury										(2,644)	(3,191)	(2,108)
Common stock issued, net										8,207	112,085	7,898
Net cash provided by financing activities										(18,494)	(10,107)	110,185
Net change in cash and cash equivalents										21,798	5,544	(14,833)
Cash and cash equivalents at beginning of year					$ 69,009				$ 63,465	69,009	63,465	78,298
Cash and cash equivalents at end of year	$ 69,009	$ 90,807				$ 69,009				$ 90,807	$ 69,009	$ 63,465