Fair Value Measurements - Measured Under Certain Conditions (Details) - USD ($)	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Impaired [Line Items]
Realized investment losses on impaired real estate, partnership entities, and commercial mortgage loans, after-tax	$ 0
Maximum [Member]
Financing Receivable, Impaired [Line Items]
Impaired real estate, partnership entities, and commercial mortgage loans as a percent of total investments		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.

Name:	us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration