Fair Value - Additional Information (Detail) (USD $)	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]
Transfers between Level 1 and Level 2	$ 0			$ 0
Level 3 assets fair value	38,000,000	37,000,000	37,000,000	37,000,000	36,000,000	38,000,000
Upper limit percentage of Level 3 assets to total invested assets	0.30%
Long-term debt, carrying value	3,826,000,000		2,600,000,000
Carrying value of long-term debt	4,338,000,000		2,600,000,000
Long-term debt, fair value	$ 4,565,000,000		$ 2,751,000,000