Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,442,222	$ 2,250,460
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	37,012	18,622
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,581	6,335
Held to Maturity, Estimated Fair Value	1,476,653	2,262,747
Available for Sale, Amortized Cost	10,051,799	10,206,176
Available for Sale, Gross Unrealized Gains	178,156	105,473
Available for Sale, Gross Unrealized Losses	44,855	108,372
Available for Sale, Estimated Fair Value	10,185,100	10,203,277
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	0	249,889
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	1,762
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held to Maturity, Estimated Fair Value	0	251,651
Available for Sale, Amortized Cost	3,452,882	4,003,692
Available for Sale, Gross Unrealized Gains	23,796	24,984
Available for Sale, Gross Unrealized Losses	3,050	8,945
Available for Sale, Estimated Fair Value	3,473,628	4,019,731
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	3,708	4,511
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	21	39
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	24	0
Held to Maturity, Estimated Fair Value	3,705	4,550
Available for Sale, Amortized Cost	658,281	756,072
Available for Sale, Gross Unrealized Gains	24,218	30,388
Available for Sale, Gross Unrealized Losses	1,304	1,293
Available for Sale, Estimated Fair Value	681,195	785,167
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,437,164	1,994,710
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	36,991	16,821
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,556	6,335
Held to Maturity, Estimated Fair Value	1,471,599	2,005,196
Available for Sale, Amortized Cost	5,898,098	5,403,918
Available for Sale, Gross Unrealized Gains	130,142	50,101
Available for Sale, Gross Unrealized Losses	40,501	98,134
Available for Sale, Estimated Fair Value	5,987,739	5,355,885
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,350	1,350
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	1	0
Held to Maturity, Estimated Fair Value	1,349	1,350
Available for Sale, Amortized Cost	42,538	42,494
Available for Sale, Gross Unrealized Gains	0	0
Available for Sale, Gross Unrealized Losses	0	0
Available for Sale, Estimated Fair Value	$ 42,538	$ 42,494

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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