Schedule of Estimated Fair Value (Detail) - USD ($) $ in Thousands	Jun. 17, 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	$ 784,000	788,000
Fair Value	797,328	821,884
2017 Five-Year Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	595,500	598,500
Fair Value	578,826	592,515
2017 Ten-Year Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	992,500	997,500
Fair Value	981,583	1,023,435
2017 Five-Year Floating Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	297,750	299,250
Fair Value	298,941	$ 300,746
2018 7.5-Year Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	425,000
Fair Value	422,875
2018 9.25-Year Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Principal Amount	400,000
Fair Value	$ 402,400