Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Cash Flows) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net income	$ 277,977us-gaap_NetIncomeLoss	$ 237,866us-gaap_NetIncomeLoss	$ 237,952us-gaap_NetIncomeLoss
Stock-based compensation	12,503us-gaap_ShareBasedCompensation	11,963us-gaap_ShareBasedCompensation	12,836us-gaap_ShareBasedCompensation
Excess tax benefits from stock-based compensation	(3,183)	(2,686)	(171)
Net cash from operating activities	286,670	173,606	299,999
Net cash received (paid) in acquisitions	830,661	(1,896)	(7,199)
Net cash from investing activities	(2,282,093)	(385,922)	(2,155,468)
Proceeds from stock option exercises	29,158	68,653	10,516
Excess tax benefits from stock-based compensation	3,183	2,686	171
Proceeds from issuance of preferred stock	0	144,486	0
Purchase of treasury stock	(1,457)	(144,630)	(436)
Cash dividends paid on preferred stock	(8,063)	(6,719)	0
Cash dividends paid on common stock	(127,178)us-gaap_PaymentsOfDividendsCommonStock	(119,619)us-gaap_PaymentsOfDividendsCommonStock	(116,853)us-gaap_PaymentsOfDividendsCommonStock
Net cash from financing activities	1,803,421	1,243,462	2,472,856
Net change in cash and cash equivalents	(192,002)	1,031,146	617,387
Cash and cash equivalents at beginning of year	4,556,125	3,524,979	2,907,592
Cash and cash equivalents at end of year		4,556,125	3,524,979
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	277,977	237,866	237,952
Equity in undistributed earnings of subsidiaries	(165,569)	(97,756)	(99,887)
Stock-based compensation	441	330	330
Excess tax benefits from stock-based compensation	(165)	(155)	(133)
Net change in other assets and other liabilities	(1,984)	2,372	(2,256)
Net cash from operating activities	110,700	142,657	136,006
Net cash received (paid) in acquisitions	830,661	0	0
Capital contribution to subsidiaries	(879,730)	0	0
Net cash from investing activities	(49,069)	0	0
Proceeds from stock option exercises	29,158	68,653	10,516
Proceeds from stock-based compensation activities of subsidiaries	12,062	11,633	12,506
Excess tax benefits from stock-based compensation	165	155	133
Proceeds from issuance of preferred stock	0	144,486	0
Purchase of treasury stock	(1,457)	(144,630)	(436)
Cash dividends paid on preferred stock	(8,063)	(6,719)	0
Cash dividends paid on common stock	(127,178)	(119,619)	(116,853)
Net cash from financing activities	(95,313)	(46,041)	(94,134)
Net change in cash and cash equivalents	(33,682)	96,616	41,872
Cash and cash equivalents at beginning of year	327,677	231,061	189,189
Cash and cash equivalents at end of year	$ 293,995	$ 327,677	$ 231,061

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Equity in undistributed earnings of subsidiaries.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of increase (decrease) in cash and cash equivalents. Cash and cash equivalents are the amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Includes effect from exchange rate changes.

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Amount of cash inflow from realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

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Amount of cash outflow for realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

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Amount of cash inflow (outflow) of financing activities, excluding discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

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Amount of cash inflow (outflow) of investing activities, excluding discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The cash outflow to reacquire common stock during the period.

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The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase.

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Proceeds from issuance of capital stock which provides for a specific dividend that is paid to the shareholders before any dividends to common stockholders and which takes precedence over common stockholders in the event of liquidation.

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The cash inflow associated with the amount received from holders to acquire the entity's shares under incentive and share awards other than stock option exercises. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

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The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

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