Fair Value and Carrying Value of Long-Term Debt (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Estimated Fair Value	$ 2,255,300,000	$ 2,188,700,000
2012 Facility	0	0
Series Two Thousand Five Dash One Notes [Member]
Debt Instrument [Line Items]
Estimated Fair Value	315,700,000	319,200,000
Notes Payable	305,800,000	310,300,000
Series Two Thousand Seven Dash One Notes [Member]
Debt Instrument [Line Items]
Estimated Fair Value	339,700,000	334,700,000
Notes Payable	300,000,000	300,000,000
Twenty Ten Senior Notes [Member]
Debt Instrument [Line Items]
Estimated Fair Value	558,000,000	536,600,000
Notes Payable	497,800,000	497,800,000
Twenty Twelve Senior Notes [Member]
Debt Instrument [Line Items]
Estimated Fair Value	516,100,000	497,000,000
Notes Payable	496,600,000	496,500,000
Twenty Thirteen Senior Notes [Member]
Debt Instrument [Line Items]
Estimated Fair Value	525,800,000	501,200,000
Notes Payable	$ 497,300,000	$ 497,200,000