Schedule of Segment Financial Results (Detail) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	[1]	Mar. 31, 2016	[1]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net interest income	$ 146,346	$ 140,858	$ 137,394	$ 134,315	$ 131,456	$ 124,765	$ 121,210		$ 117,892		$ 558,913	$ 495,323	$ 412,067
Provision for loan losses	6,000	11,500	14,500	9,000	7,500	13,000	7,000		5,000		41,000	32,500	15,500
Noninterest income	106,033	104,306	110,306	102,917	98,230	103,542	102,774		97,965		423,562	402,511	370,659
Noninterest expense	182,559	171,821	176,939	173,810	167,847	165,211	168,419		165,268		705,129	666,745	638,938
Income before income taxes											236,346	198,589	128,288
Income tax expense	16,463	12,971	11,490	12,446	10,939	10,674	12,102		11,240		53,370	44,955	31,730
Income from continuing operations	$ 47,357	$ 48,872	$ 44,771	$ 41,976	$ 43,400	$ 39,422	$ 36,463		$ 34,349		182,976	153,634	96,558
Average assets											20,396,000	19,593,000	17,786,000
Bank [Member]
Segment Reporting Information [Line Items]
Net interest income											546,000	484,716	406,884
Provision for loan losses											41,000	32,500	15,500
Noninterest income											328,550	309,889	274,376
Noninterest expense											616,883	582,719	555,962
Income before income taxes											216,667	179,386	109,798
Income tax expense											49,522	40,406	26,651
Income from continuing operations											167,145	138,980	83,147
Average assets											19,612,450	18,371,950	16,801,000
Asset Servicing [Member]
Segment Reporting Information [Line Items]
Net interest income											12,913	10,607	5,183
Noninterest income											95,012	92,622	96,283
Noninterest expense											88,246	84,026	82,976
Income before income taxes											19,679	19,203	18,490
Income tax expense											3,848	4,549	5,079
Income from continuing operations											15,831	14,654	13,411
Average assets											$ 783,550	$ 1,221,050	$ 985,000

[1] During the third quarter of 2016, the Company early adopted ASU No. 2016-09 with an effective date of January 1, 2016. As part of the adoption of this standard, the Company made an accounting policy election to account for stock compensation forfeitures on an actual basis and discontinue the use of an estimated forfeiture approach. This change required a modified retrospective adoption, via a cumulative effect adjustment and recasting of first quarter and second quarter 2016 operating results. The impact of this adoption was an increase to net income of $158 thousand and $220 thousand for the first and second quarters, respectively. Additionally, basic and diluted net income per share increased $0.01 for both periods.