Investments (details) - Derivatives - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Futures [Member] \| U.S. Treasury notes contracts [Member]
Derivatives
Notional value of open contracts	$ 400	$ 400

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant