Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Gross Unrealized Losses
Less than twelve months	$ (19,498)	$ (44,874)
Over twelve months	(12,593)	(2,677)
Total	(32,091)	(47,551)
Estimated Fair Value
Less than twelve months	2,568,953	2,526,660
Over twelve months	544,383	102,762
Total	3,113,336	2,629,422
Gross Unrealized Losses
Less than twelve months	(476)	(463)
Over twelve months	(575)	(487)
Total	(1,051)	(950)
Estimated Fair Value
Less than twelve months	87,552	22,890
Over twelve months	22,148	10,960
Total	109,700	33,850
U.S. Government-Sponsored Enterprise Obligations
Gross Unrealized Losses
Less than twelve months	(171)	(549)
Over twelve months	0	0
Total	(171)	(549)
Estimated Fair Value
Less than twelve months	54,825	150,554
Over twelve months	0	0
Total	54,825	150,554
Obligations of State and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	(204)	(5,207)
Over twelve months	(1,312)	0
Total	(1,516)	(5,207)
Estimated Fair Value
Less than twelve months	29,563	148,059
Over twelve months	68,827	0
Total	98,390	148,059
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(18,703)	(38,667)
Over twelve months	(11,265)	(2,624)
Total	(29,968)	(41,291)
Estimated Fair Value
Less than twelve months	2,416,882	2,191,563
Over twelve months	474,232	98,912
Total	2,891,114	2,290,475
Other Securities
Gross Unrealized Losses
Less than twelve months	(420)	(451)
Over twelve months	(16)	(53)
Total	(436)	(504)
Estimated Fair Value
Less than twelve months	67,683	36,484
Over twelve months	1,324	3,850
Total	69,007	40,334
Obligations of State and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	(456)	(133)
Over twelve months	(27)	0
Total	(483)	(133)
Estimated Fair Value
Less than twelve months	85,456	10,602
Over twelve months	3,031	0
Total	88,487	10,602
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(20)	(330)
Over twelve months	(548)	(487)
Total	(568)	(817)
Estimated Fair Value
Less than twelve months	2,096	12,288
Over twelve months	19,117	10,960
Total	$ 21,213	$ 23,248