Debt - Optional Redemptions - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017
Disclosure of detailed information about borrowings [abstract]
Redemption description	All of our outstanding notes, except the 2024 notes, are redeemable at any time by repaying the greater of the principal amount and the present value of the sum of the remaining scheduled principal and interest amounts discounted at a comparable treasury yield plus a stipulated spread, plus, in each case, accrued interest to, but not including, the date of redemption. In addition, the 2023, 2042 and 2043 notes issued in 2012 are callable at 100% (plus accrued interest to, but not including, the date of redemption) at any time on or after November 1, 2022, September 1, 2041, and August 1, 2042, respectively. The 2022 and 2041 notes issued in 2011 are callable at 100% at any time on or after October 15, 2021, and January 15, 2041, respectively. The January 2021 notes are callable at 100% on or after October 15, 2020, and the 2040 notes are callable at 100% on or after February 15, 2040.

Debt - Optional Redemptions - Additional Information (Detail)

12 Months Ended

Dec. 31, 2017

Disclosure of detailed information about borrowings [abstract]

All of our outstanding notes, except the 2024 notes, are redeemable at any time by repaying the greater of the principal amount and the present value of the sum of the remaining scheduled principal and interest amounts discounted at a comparable treasury yield plus a stipulated spread, plus, in each case, accrued interest to, but not including, the date of redemption. In addition, the 2023, 2042 and 2043 notes issued in 2012 are callable at 100% (plus accrued interest to, but not including, the date of redemption) at any time on or after November 1, 2022, September 1, 2041, and August 1, 2042, respectively. The 2022 and 2041 notes issued in 2011 are callable at 100% at any time on or after October 15, 2021, and January 15, 2041, respectively. The January 2021 notes are callable at 100% on or after October 15, 2020, and the 2040 notes are callable at 100% on or after February 15, 2040.