Summary of Total Indebtedness (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
2012 Facility	$ 0	$ 0
Total long-term debt	2,536,500,000	2,101,800,000
Series Two Thousand Five Dash One Notes [Member]
Debt Instrument [Line Items]
Notes Payable		310,300,000
Series Two Thousand Seven Dash One Notes [Member]
Debt Instrument [Line Items]
Notes Payable	300,000,000	300,000,000
Twenty Ten Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	495,900,000	497,800,000
Twenty Twelve Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	496,800,000	496,500,000
Twenty Thirteen Senior Notes [Member]
Debt Instrument [Line Items]
Notes Payable	497,400,000	497,200,000
Two Thousand Fourteen Senior Notes Five Year [Member]
Debt Instrument [Line Items]
Notes Payable	448,000,000	0
Two Thousand Fourteen Senior Notes Thirty Year [Member]
Debt Instrument [Line Items]
Notes Payable	$ 298,400,000	$ 0