Derivative Financial Instruments - Information about Hedged Items Included in Consolidated Balance Sheet (Detail) - Long-term Debt [Member] - Derivatives Designated and Qualifying as Hedging Instruments [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Carrying Amount [Member]
Derivatives, Fair Value [Line Items]
Hedged Item	$ 5,085,610	$ 4,504,029
Cumulative Amount of Fair Value Hedging Adjustment Increasing (Decreasing) the Carrying Amount [Member]
Derivatives, Fair Value [Line Items]
Hedged Item	$ (108,994)	$ (40,133)