Derivatives and Hedging Activities - Interest Rate Derivatives Outstanding Designated as Cash Flow Hedges (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Interest Rate Derivatives Outstanding
Notional amount	$ 10,110,532,000	$ 9,250,352,000
Interest Rate Swaps
Interest Rate Derivatives Outstanding
Notional amount	$ 9,270,000,000	8,620,000,000
Fixed Interest Rate in Contract	2.22%
Interest Rate Swaps | UBS AG
Interest Rate Derivatives Outstanding
Notional amount	$ 500,000,000
Fixed Interest Rate in Contract	1.102%
Interest Rate Swaps | ING Capital Markets LLC
Interest Rate Derivatives Outstanding
Notional amount	$ 400,000,000
Fixed Interest Rate in Contract	0.8675%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 300,000,000
Fixed Interest Rate in Contract	2.496%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 300,000,000
Fixed Interest Rate in Contract	2.691%
Interest Rate Swaps | Wells Fargo Bank, N.A.
Interest Rate Derivatives Outstanding
Notional amount	$ 200,000,000
Fixed Interest Rate in Contract	3.142%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 500,000,000
Fixed Interest Rate in Contract	2.249%
Interest Rate Swaps | Citibank, N.A.
Interest Rate Derivatives Outstanding
Notional amount	$ 200,000,000
Fixed Interest Rate in Contract	2.83%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 500,000,000
Fixed Interest Rate in Contract	2.439%
Interest Rate Swaps | HSBC Bank USA, National Association
Interest Rate Derivatives Outstanding
Notional amount	$ 550,000,000
Fixed Interest Rate in Contract	2.45%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 1,000,000,000
Fixed Interest Rate in Contract	2.213%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 1,000,000,000
Fixed Interest Rate in Contract	1.8665%
Interest Rate Swaps | The Royal Bank of Scotland Plc
Interest Rate Derivatives Outstanding
Notional amount	$ 500,000,000
Fixed Interest Rate in Contract	1.98375%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 600,000,000
Fixed Interest Rate in Contract	2.883%
Interest Rate Swaps | HSBC Bank USA, National Association
Interest Rate Derivatives Outstanding
Notional amount	$ 500,000,000
Fixed Interest Rate in Contract	2.201%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 450,000,000
Fixed Interest Rate in Contract	2.0999%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 450,000,000
Fixed Interest Rate in Contract	2.1456%
Interest Rate Swaps | LCH Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 1,000,000,000
Fixed Interest Rate in Contract	2.7735%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 100,000,000
Fixed Interest Rate in Contract	2.0375%
Interest Rate Swaps | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 220,000,000
Fixed Interest Rate in Contract	2.1225%
Interest Rate Swaps with Forward Start Dates
Interest Rate Derivatives Outstanding
Notional amount	$ 1,300,000,000	$ 500,000,000
Fixed Interest Rate in Contract	2.63%
Interest Rate Swaps with Forward Start Dates | LCH Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 600,000,000
Fixed Interest Rate in Contract	2.6405%
Interest Rate Swaps with Forward Start Dates | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 250,000,000
Fixed Interest Rate in Contract	2.778%
Interest Rate Swaps with Forward Start Dates | CME Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 250,000,000
Fixed Interest Rate in Contract	2.387%
Interest Rate Swaps with Forward Start Dates | LCH Central Clearing
Interest Rate Derivatives Outstanding
Notional amount	$ 200,000,000
Fixed Interest Rate in Contract	2.741%