Mortgage Loans on Real Estate Schedule of Mortgage Loan Portfolio by Geographic Region and Property Type (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 2,600,000
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	2,625,737	2,658,883
Percent of Mortgage Loans	100.00%	100.00%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Office Property Type [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	684,553	666,467
Percent of Mortgage Loans	26.10%	25.10%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Medical Office Property Type [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	130,874	136,764
Percent of Mortgage Loans	5.00%	5.10%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Retail Property Type [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	656,187	677,951
Percent of Mortgage Loans	25.00%	25.50%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Industrial and Warehouse Property Type [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	672,522	692,637
Percent of Mortgage Loans	25.60%	26.10%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Hotel Property Type [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	89,376	94,045
Percent of Mortgage Loans	3.40%	3.50%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Apartment Property Type [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	220,698	219,335
Percent of Mortgage Loans	8.40%	8.20%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Mixed Use or Other Property Type [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	171,527	171,684
Percent of Mortgage Loans	6.50%	6.50%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans East United States Region [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	744,863	732,762
Percent of Mortgage Loans	28.40%	27.50%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Middle Atlantic United States Region [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	163,497	155,094
Percent of Mortgage Loans	6.20%	5.80%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Mountain United States Region [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	371,550	387,599
Percent of Mortgage Loans	14.20%	14.60%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans New England United States Region [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	24,675	26,385
Percent of Mortgage Loans	0.90%	1.00%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans Pacific United States Region [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	317,017	320,982
Percent of Mortgage Loans	12.10%	12.10%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans South Atlantic United States Region [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	459,579	458,802
Percent of Mortgage Loans	17.50%	17.30%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans West North Central United States Region [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	354,299	370,168
Percent of Mortgage Loans	13.50%	13.90%
Commercial Real Estate Portfolio Segment [Member] \| Mortgage Loans West South Central United States Region [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Real Estate	$ 190,257	$ 207,091
Percent of Mortgage Loans	7.20%	7.80%

Amount before allowance of commercial loans issued to businesses to acquire, develop, construct, improve, or refinance land or building. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans covered under loss sharing agreements.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph a

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(a))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5066-111524

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1(c))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 01-6

-Paragraph 13

-Subparagraph e

-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.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 60

-Paragraph 47

-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.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Paragraph 51, 52, 53

-IssueDate 2006-05-01

-Chapter 8

-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.

+ Details