Fair Value Measurements - Measured Under Certain Conditions (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Realized investment losses on impaired real estate, partnership entities, and commercial mortgage loans, after-tax
Maximum [Member]
Financing Receivable, Impaired [Line Items]
Impaired real estate, partnership entities, and commercial mortgage loans as a percent of total investments	1.00%	1.00%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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