Notional Amounts of Swap Agreement, Weighted Average Interest Rates and Remaining Term (Detail) - Interest rate swap agreements - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Notional Amount	$ 3,846,000,000	$ 3,330,000,000
Weighted Average Fixed Rate	1.30%	1.56%
Remaining Term in Months	35 months	48 months
Less than 12 months
Derivative [Line Items]
Notional Amount	$ 400,000,000	$ 0
Weighted Average Fixed Rate	0.67%	0.00%
Remaining Term in Months	10 months	0 months
1 year to 2 years
Derivative [Line Items]
Notional Amount	$ 1,175,000,000	$ 550,000,000
Weighted Average Fixed Rate	0.87%	0.80%
Remaining Term in Months	17 months	20 months
2 years to 3 years
Derivative [Line Items]
Notional Amount	$ 915,000,000	$ 795,000,000
Weighted Average Fixed Rate	1.14%	1.03%
Remaining Term in Months	30 months	31 months
3 years to 5 years
Derivative [Line Items]
Notional Amount	$ 656,000,000	$ 1,080,000,000
Weighted Average Fixed Rate	1.36%	1.48%
Remaining Term in Months	44 months	46 months
5 years to 7 years
Derivative [Line Items]
Notional Amount	$ 430,000,000	$ 555,000,000
Weighted Average Fixed Rate	2.39%	2.35%
Remaining Term in Months	73 months	72 months
7 years to 10 years
Derivative [Line Items]
Notional Amount	$ 270,000,000	$ 350,000,000
Weighted Average Fixed Rate	2.67%	2.93%
Remaining Term in Months	93 months	100 months