Assets and Liabilities Measured at Fair Value Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	$ 72,588	$ 55,764
Available for sale securities	6,739,736	6,802,656
Contingent consideration liability	49,442	51,163	57,423	72,046
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	72,588	55,764
Available for sale securities	6,841,584	6,937,463
Company-owned life insurance	11,144	10,539
Derivatives	2,870	3,503
Total	6,928,186	7,007,269
Deferred compensation	16,172	13,705
Contingent consideration liability	49,442	51,163
Derivatives	2,886	3,625
Total	68,500	68,493
Fair Value, Measurements, Recurring [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	1,733	400
Available for sale securities	122,821	117,851
Fair Value, Measurements, Recurring [Member] | U.S. Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	559	506
Available for sale securities	1,048,972	1,026,115
Fair Value, Measurements, Recurring [Member] | Mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	33,090	11,288
Available for sale securities	3,270,718	3,556,193
Fair Value, Measurements, Recurring [Member] | State and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	5,377	12,913
Available for sale securities	1,969,936	1,892,684
Fair Value, Measurements, Recurring [Member] | Trading - Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	31,829	30,657
Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities	429,137	338,887
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities		5,733
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	33,543	31,057
Available for sale securities	551,958	456,738
Total	585,501	487,795
Deferred compensation	16,172	13,705
Total	16,172	13,705
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	1,733	400
Available for sale securities	122,821	117,851
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Trading - Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	31,810	30,657
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities	429,137	338,887
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	39,045	24,707
Available for sale securities	6,289,626	6,480,725
Company-owned life insurance	11,144	10,539
Derivatives	2,870	3,503
Total	6,342,685	6,519,474
Derivatives	2,886	3,625
Total	2,886	3,625
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	559	506
Available for sale securities	1,048,972	1,026,115
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	33,090	11,288
Available for sale securities	3,270,718	3,556,193
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | State and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	5,377	12,913
Available for sale securities	1,969,936	1,892,684
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Trading - Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	19
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities		5,733
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total
Contingent consideration liability	49,442	51,163
Total	$ 49,442	$ 51,163