Business Segments - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Interest income, including loan fees	$ 68,015	$ 64,515	$ 132,515	$ 128,695
Credit for funds provided	0	0	0	0
Total interest income	68,015	64,515	132,515	128,695
Interest expense	2,059	1,757	4,043	4,928
Charge for funds used	0	0	0	0
Total interest expense	2,059	1,757	4,043	4,928
Net interest income	65,956	62,758	128,472	123,767
Recapture of provision for loan losses	0	(2,000)	0	(2,000)
Net interest income after recapture of provision for loan losses	65,956	64,758	128,472	125,767
Noninterest income	9,274	8,345	17,957	16,356
Noninterest expense	34,422	31,533	68,786	62,135
Debt termination expense	16	0	16	13,870
Earnings before income taxes	40,792	41,570	77,627	66,118
Segment assets	8,312,307	7,697,358	8,312,307	7,697,358	$ 7,671,200
Operating Segments [Member] | Centers [Member]
Segment Reporting Information [Line Items]
Interest income, including loan fees	38,953	35,813	75,457	71,181
Credit for funds provided	8,820	8,530	17,517	16,741
Total interest income	47,773	44,343	92,974	87,922
Interest expense	1,723	1,628	3,403	3,291
Charge for funds used	1,467	1,052	2,754	2,119
Total interest expense	3,190	2,680	6,157	5,410
Net interest income	44,583	41,663	86,817	82,512
Recapture of provision for loan losses	0	0	0	0
Net interest income after recapture of provision for loan losses	44,583	41,663	86,817	82,512
Noninterest income	5,326	5,319	10,153	10,386
Noninterest expense	12,891	12,259	25,501	24,108
Debt termination expense	0	0	0	0
Earnings before income taxes	37,018	34,723	71,469	68,790
Segment assets	6,967,395	6,436,216	6,967,395	6,436,216
Operating Segments [Member] | Treasury [Member]
Segment Reporting Information [Line Items]
Interest income, including loan fees	17,779	19,210	36,536	37,865
Credit for funds provided	0	0	0	0
Total interest income	17,779	19,210	36,536	37,865
Interest expense	203	31	387	1,462
Charge for funds used	15,629	15,441	30,978	30,247
Total interest expense	15,832	15,472	31,365	31,709
Net interest income	1,947	3,738	5,171	6,156
Recapture of provision for loan losses	0	0	0	0
Net interest income after recapture of provision for loan losses	1,947	3,738	5,171	6,156
Noninterest income	0	0	0	0
Noninterest expense	218	211	434	424
Debt termination expense	16	0	16	13,870
Earnings before income taxes	1,713	3,527	4,721	(8,138)
Segment assets	3,738,321	3,624,321	3,738,321	3,624,321
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Interest income, including loan fees	11,283	9,492	20,522	19,649
Credit for funds provided	14,004	13,024	27,685	25,665
Total interest income	25,287	22,516	48,207	45,314
Interest expense	133	98	253	175
Charge for funds used	5,728	5,061	11,470	10,040
Total interest expense	5,861	5,159	11,723	10,215
Net interest income	19,426	17,357	36,484	35,099
Recapture of provision for loan losses	0	(2,000)	0	(2,000)
Net interest income after recapture of provision for loan losses	19,426	19,357	36,484	37,099
Noninterest income	3,948	3,026	7,804	5,970
Noninterest expense	21,313	19,063	42,851	37,603
Debt termination expense	0	0	0	0
Earnings before income taxes	2,061	3,320	1,437	5,466
Segment assets	943,289	875,585	943,289	875,585
Eliminations [Member]
Segment Reporting Information [Line Items]
Interest income, including loan fees	0	0	0	0
Credit for funds provided	(22,824)	(21,554)	(45,202)	(42,406)
Total interest income	(22,824)	(21,554)	(45,202)	(42,406)
Interest expense	0	0	0	0
Charge for funds used	(22,824)	(21,554)	(45,202)	(42,406)
Total interest expense	(22,824)	(21,554)	(45,202)	(42,406)
Net interest income	0	0	0	0
Recapture of provision for loan losses	0	0	0	0
Net interest income after recapture of provision for loan losses	0	0	0	0
Noninterest income	0	0	0	0
Noninterest expense	0	0	0	0
Debt termination expense	0	0	0	0
Earnings before income taxes	0	0	0	0
Segment assets	$ (3,336,698)	$ (3,238,764)	$ (3,336,698)	$ (3,238,764)