Schedule of Estimated Fair Value (Detail) - USD ($) $ in Thousands	Sep. 09, 2018	Dec. 31, 2017
2015 Class Five-Year Notes [Member]
Debt Instrument [Line Items]
Principal Amount		$ 492,500
Fair Value		494,470
2015 Ten-Year Notes [Member]
Debt Instrument [Line Items]
Principal Amount	$ 782,000	788,000
Fair Value	792,166	821,884
2017 Five-Year Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	594,000	598,500
Fair Value	576,180	592,515
2017 Ten-Year Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	990,000	997,500
Fair Value	978,120	1,023,435
2017 Five-Year Floating Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	297,000	299,250
Fair Value	298,188	$ 300,746
2018 7.5-Year Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	423,938
Fair Value	421,394
2018 9.25-Year Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	399,000
Fair Value	$ 398,202