Investments (details) - Derivatives - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Futures [Member] \| U.S. Treasury notes contracts [Member]
Derivatives
Notional value of open contracts	$ 450	$ 350

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