Derivatives and Hedging Activities - Interest Rate Derivatives Outstanding (Detail) - USD ($)		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Interest Rate Derivatives Outstanding
Notional amount		$ 11,826,324,000	$ 11,961,088,000
Interest Rate Swaps
Interest Rate Derivatives Outstanding
Notional amount		$ 11,450,000,000	$ 10,550,000,000
Fixed interest rate in contract		2.12%
Interest Rate Swaps | Credit Suisse International Maturing 4/15/2016
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		Apr. 15, 2016
Fixed interest rate in contract		2.27%
Interest Rate Swaps | Bank of New York Mellon Maturing 4/15/2016
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		Apr. 15, 2016
Fixed interest rate in contract		2.24%
Interest Rate Swaps | JP Morgan Chase Bank, N.A. Maturing 5/16/2016
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		May 16, 2016
Fixed interest rate in contract		2.31%
Interest Rate Swaps | Goldman Sachs Bank USA Maturing 5/24/2016
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		May 24, 2016
Fixed interest rate in contract		2.34%
Interest Rate Swaps | Goldman Sachs Bank USA Maturing 6/15/2016
Interest Rate Derivatives Outstanding
Notional amount		$ 250,000,000
Maturity date		Jun. 15, 2016
Fixed interest rate in contract		2.67%
Interest Rate Swaps | Wells Fargo Bank, N.A. Maturing 6/15/2016
Interest Rate Derivatives Outstanding
Notional amount		$ 250,000,000
Maturity date		Jun. 15, 2016
Fixed interest rate in contract		2.67%
Interest Rate Swaps | JPMorgan Chase Bank, N.A. Maturing 6/24/2016
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		Jun. 24, 2016
Fixed interest rate in contract		2.51%
Interest Rate Swaps | Citibank, N.A Maturing 10/15/2016
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		Oct. 15, 2016
Fixed interest rate in contract		1.93%
Interest Rate Swaps | Deutsche Bank AG Maturing 2/5/2018
Interest Rate Derivatives Outstanding
Notional amount		$ 150,000,000
Maturity date		Feb. 05, 2018
Fixed interest rate in contract		2.90%
Interest Rate Swaps | ING Capital Markets LLC Maturing 2/24/2018
Interest Rate Derivatives Outstanding
Notional amount		$ 350,000,000
Maturity date		Feb. 24, 2018
Fixed interest rate in contract		0.95%
Interest Rate Swaps | ING Capital Markets LLC Maturing 5/5/2018
Interest Rate Derivatives Outstanding
Notional amount		$ 300,000,000
Maturity date		May 05, 2018
Fixed interest rate in contract		0.79%
Interest Rate Swaps | UBS AG Maturing 5/24/2018
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		May 24, 2018
Fixed interest rate in contract		1.10%
Interest Rate Swaps | ING Captial Markets LLC Maturing 6/5/2018
Interest Rate Derivatives Outstanding
Notional amount		$ 400,000,000
Maturity date		Jun. 05, 2018
Fixed interest rate in contract		0.87%
Interest Rate Swaps | The Royal Bank of Scotland Plc Maturing 9/5/2018
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		Sep. 05, 2018
Fixed interest rate in contract		1.04%
Interest Rate Swaps | Citibank, N.A. CME Clearing House Maturing 2/5/2021
Interest Rate Derivatives Outstanding
Notional amount	[1]	$ 300,000,000
Maturity date	[1]	Feb. 05, 2021
Fixed interest rate in contract	[1]	2.50%
Interest Rate Swaps | The Royal Bank of Scotland Plc CME Clearing House Maturing 2/5/2021
Interest Rate Derivatives Outstanding
Notional amount	[1]	$ 300,000,000
Maturity date	[1]	Feb. 05, 2021
Fixed interest rate in contract	[1]	2.69%
Interest Rate Swaps | Wells Fargo Bank, N.A. Maturing 3/15/2021
Interest Rate Derivatives Outstanding
Notional amount		$ 200,000,000
Maturity date		Mar. 15, 2021
Fixed interest rate in contract		3.14%
Interest Rate Swaps | JP Morgan Chase Bank, N.A. CME Central Clearing Maturing 5/24/2021
Interest Rate Derivatives Outstanding
Notional amount	[2]	$ 500,000,000
Maturity date	[2]	May 24, 2021
Fixed interest rate in contract	[2]	2.25%
Interest Rate Swaps | Citibank, N.A. Maturing 5/25/2021
Interest Rate Derivatives Outstanding
Notional amount		$ 200,000,000
Maturity date		May 25, 2021
Fixed interest rate in contract		2.83%
Interest Rate Swaps | HSBC Bank USA, National Association CME Central Clearing, Maturing 6/24/2021
Interest Rate Derivatives Outstanding
Notional amount	[3]	$ 500,000,000
Maturity date	[3]	Jun. 24, 2021
Fixed interest rate in contract	[3]	2.44%
Interest Rate Swaps | HSBC Bank USA, National Association Maturing 2/24/2022
Interest Rate Derivatives Outstanding
Notional amount		$ 550,000,000
Maturity date		Feb. 24, 2022
Fixed interest rate in contract		2.45%
Interest Rate Swaps | Deutsche Bank AG CME Central Clearing, Maturing 6/9/2022
Interest Rate Derivatives Outstanding
Notional amount		$ 1,000,000,000
Maturity date		Jun. 09, 2022
Fixed interest rate in contract		2.21%
Interest Rate Swaps | HSBC Bank USA, National Association Maturing 6/5/2023
Interest Rate Derivatives Outstanding
Notional amount		$ 250,000,000
Maturity date		Jun. 05, 2023
Fixed interest rate in contract		1.91%
Interest Rate Swaps | Royal Bank Of Scotland Plc Maturing 8/15/2023
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		Aug. 15, 2023
Fixed interest rate in contract		1.98%
Interest Rate Swaps | Goldman Sachs Bank USA CME Clearing House Maturing 8/24/2023
Interest Rate Derivatives Outstanding
Notional amount		$ 600,000,000
Maturity date		Aug. 24, 2023
Fixed interest rate in contract		2.88%
Interest Rate Swaps | UBS AG Maturing 11/15/2023
Interest Rate Derivatives Outstanding
Notional amount		$ 250,000,000
Maturity date		Nov. 15, 2023
Fixed interest rate in contract		2.23%
Interest Rate Swaps | HSBC Bank USA National Association Maturing 12/15/2023
Interest Rate Derivatives Outstanding
Notional amount		$ 500,000,000
Maturity date		Dec. 15, 2023
Fixed interest rate in contract		2.20%
Interest Rate Swaps | Morgan Stanley Capital Services, LLC Maturing 4/2/2025
Interest Rate Derivatives Outstanding
Notional amount		$ 100,000,000
Maturity date		Apr. 02, 2025
Fixed interest rate in contract		2.04%

[1] Forward start date of February 2016

[2] Forward start date of May 2016

[3] Forward start date of June 2016