Fair Value Measurements Narrative (Details) (Investment Segment [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Risk premium adjustment assumption to project change in fair value of loan	1.00%
Change in fair value of loan from 1 percent change in risk premium	2.00%
Level 3 [Member]
Fair Value, Instruments Classified in Shareholders' Equity Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unrealized losses included in earnings related to Level 3 investments	4

This item represents the amount of the total change in unrealized (holding) gains or losses for the period which are included in the statement of income (or changes in net assets) in investment income. Such unrealized (holding) gains or losses relate to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration