Fair Value - Additional Information (Detail) - USD ($)	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Transfers between Level 1 and Level 2	$ 0			$ 0
Level 3 assets fair value	$ 11,000,000	$ 12,000,000	$ 32,000,000	$ 37,000,000	$ 37,000,000	$ 37,000,000
Upper limit percentage of Level 3 assets to total invested assets	0.10%
Carrying value of long-term debt	$ 3,823,000,000		3,825,000,000
Long-term debt, fair value	$ 3,967,000,000		$ 4,102,000,000