Securities (Year End Securities Held to Maturity and Available for Sale) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 2,926,486us-gaap_HeldToMaturitySecurities	$ 3,139,748us-gaap_HeldToMaturitySecurities
Held to Maturity, Gross Unrealized Gains	48,939	27,919
Held to Maturity, Gross Unrealized Losses	9,035	138,004
Held to Maturity, Estimated Fair Value	2,966,390	3,029,663
Available for Sale, Amortized Cost	8,261,947	5,799,135
Available for Sale, Gross Unrealized Gains	203,121	105,276
Available for Sale, Gross Unrealized Losses	3,814	8,975
Available for Sale, Estimated Fair Value	8,461,254us-gaap_AvailableForSaleSecurities	5,895,436us-gaap_AvailableForSaleSecurities
US Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	249,009	248,592
Held to Maturity, Gross Unrealized Gains	14,604	20,139
Held to Maturity, Gross Unrealized Losses	0	0
Held to Maturity, Estimated Fair Value	263,613	268,731
Available for Sale, Amortized Cost	3,783,899	2,522,159
Available for Sale, Gross Unrealized Gains	30,594	18,395
Available for Sale, Gross Unrealized Losses	3,241	0
Available for Sale, Estimated Fair Value	3,811,252	2,540,554
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	8,012	9,674
Held to Maturity, Gross Unrealized Gains	92	89
Held to Maturity, Gross Unrealized Losses	0	143
Held to Maturity, Estimated Fair Value	8,104	9,620
Available for Sale, Amortized Cost	1,331,114	1,710,664
Available for Sale, Gross Unrealized Gains	68,027	66,791
Available for Sale, Gross Unrealized Losses	417	1,439
Available for Sale, Estimated Fair Value	1,398,724	1,776,016
US Government Corporations and Agencies Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Amortized Cost	0	54,024
Available for Sale, Gross Unrealized Gains	0	0
Available for Sale, Gross Unrealized Losses	0	44
Available for Sale, Estimated Fair Value	0	53,980
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	2,668,115	2,880,482
Held to Maturity, Gross Unrealized Gains	34,243	7,691
Held to Maturity, Gross Unrealized Losses	9,035	137,861
Held to Maturity, Estimated Fair Value	2,693,323	2,750,312
Available for Sale, Amortized Cost	3,104,563	1,476,316
Available for Sale, Gross Unrealized Gains	104,500	20,090
Available for Sale, Gross Unrealized Losses	156	7,492
Available for Sale, Estimated Fair Value	3,208,907	1,488,914
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,350	1,000
Held to Maturity, Gross Unrealized Gains	0	0
Held to Maturity, Gross Unrealized Losses	0	0
Held to Maturity, Estimated Fair Value	1,350	1,000
Available for Sale, Amortized Cost	42,371	35,972
Available for Sale, Gross Unrealized Gains	0	0
Available for Sale, Gross Unrealized Losses	0	0
Available for Sale, Estimated Fair Value	$ 42,371	$ 35,972

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

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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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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Fair value of investments in debt securities classified as held-to-maturity.

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Amount of unrecognized gain before loss on investments in held-to-maturity securities.

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Amount of unrecognized loss before gain on investments in held-to-maturity securities. Excludes write-down for other than temporary impairment (OTTI).

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