Derivative Instruments and Other Hedging Activities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative Assets - Notional Amount	$ 4,713,962	$ 3,888,959
Derivative Liabilities - Notional Amount	3,862,481	3,313,072
Cash posted as variation margin for derivatives	226,800	79,600
Designated as Hedging Instruments
Derivative [Line Items]
Derivative Assets - Notional Amount	800,000	800,000
Derivative Liabilities - Notional Amount	108,500	108,500
Designated as Hedging Instruments | Interest rate contracts
Derivative [Line Items]
Derivative Assets - Notional Amount	800,000	800,000
Derivative Liabilities - Notional Amount	108,500	108,500
Not Designated as Hedging Instruments
Derivative [Line Items]
Derivative Assets - Notional Amount	3,913,962	3,088,959
Derivative Liabilities - Notional Amount	3,753,981	3,204,572
Not Designated as Hedging Instruments | Customer swaps - upstream
Derivative [Line Items]
Derivative Assets - Notional Amount	48,215	430,808
Derivative Liabilities - Notional Amount	2,855,442	2,237,542
Not Designated as Hedging Instruments | Interest rate contracts
Derivative [Line Items]
Derivative Assets - Notional Amount	2,855,442	2,237,542
Derivative Liabilities - Notional Amount	48,215	430,808
Not Designated as Hedging Instruments | Forward sales contracts
Derivative [Line Items]
Derivative Assets - Notional Amount	196,388	44,011
Derivative Liabilities - Notional Amount	463,108	230,998
Not Designated as Hedging Instruments | Written and purchased options
Derivative [Line Items]
Derivative Assets - Notional Amount	699,162	268,590
Derivative Liabilities - Notional Amount	136,043	121,981
Not Designated as Hedging Instruments | Other contracts
Derivative [Line Items]
Derivative Assets - Notional Amount	114,755	108,008
Derivative Liabilities - Notional Amount	$ 251,173	$ 183,243