Fair Value - Summary of Unobservable Inputs for the Embedded Derivatives of Interest Sensitive Contract Liabilities (Details) $ in Millions	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Non-performance risk | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Embedded derivative liability, measurement input	0.002	0.002
Non-performance risk | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Embedded derivative liability, measurement input	0.013	0.012
Option budget | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Embedded derivative liability, measurement input	0.008	0.007
Option budget | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Embedded derivative liability, measurement input	0.037	0.037
Surrender rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Embedded derivative liability, measurement input	0.044	0.015
Surrender rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Embedded derivative liability, measurement input	0.076	0.194
Embedded derivatives | Interest sensitive contract liabilities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities measured at fair value	$ 8,656	$ 7,436