This element represents a portion of states, municipalities and political subdivision securities that are advance refunded or escrowed-to-maturity securities ( collectively referred to as pre-refunded securities), which are securities for which an irrevocable trust has been established to fund the remaining payment of principal and interest.

No definition available.

Investments (Narrative) (Details)		12 Months Ended
Investments (Narrative) (Details)	May 31, 2016 USD ($)	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($) security	Dec. 31, 2014 USD ($)
Investment [Line Items]
Maximum individual state exposure		0.50%	0.50%
Advance refunded or escrowed-to-maturity securities		$ 215,275,000	$ 319,654,000
Percentage of revenue securities		46.00%	50.00%
Non-income producing material investments		$ 0	$ 0	$ 0
Securities traded continuously at a price below book value, months		6 months
Total other-than-temporary impairment		$ 6,537,000	7,212,000	69,000
Other-than-temporary impairment recognized in earnings		6,874,000	5,024,000	30,000
Other-than-temporary impairment losses recorded as a component of AOCI		$ (337,000)	$ 2,188,000	39,000
Percentage of securities representing gross unrealized losses		2.00%	5.00%
Percentage of gross unrealized losses in a continuous loss position less than twelve months		89.00%	88.00%
Individual securities comprising total gross unrealized losses (in shares) \| security		796	884
Approximate percentage, outstanding principal balance of commercial mortgage loans		34.00%
Outstanding balance of commercial mortgage loans		$ 626,337,000	$ 1,153,838,000
Commercial mortgage loan valuation allowance		2,304,000	2,582,000
Loan valuation allowance decrease		278,000	817,000
Mortgage loan commitments outstanding		7,500,000
Short term investments and fixed maturities		437,443,000	441,851,000
Certain commercial mortgage loans transferred	$ 259,741,000
Proceeds from securitization	269,828,000
Realized gain on securitization	9,092,000
Fair value of securities purchased	$ 30,822,000
Maximum loss exposure		27,762,000
Maximum exposure to loss related to VIEs		274,552,000
Unfunded commitments		62,428,000
Minimum
Investment [Line Items]
Outstanding balance of commercial mortgage loans		37,000	17,000
Maximum
Investment [Line Items]
Outstanding balance of commercial mortgage loans		$ 12,609,000	$ 14,625,000
Canadian Government/Provincials
Investment [Line Items]
Percentage of investments in foreign governments fixed maturity securities		78.00%	79.00%
Governments Of Brazil
Investment [Line Items]
Percentage of investments in foreign governments fixed maturity securities		11.00%	8.00%
Governments Of Germany
Investment [Line Items]
Percentage of investments in foreign governments fixed maturity securities		4.00%	5.00%
Other Country
Investment [Line Items]
Percentage of investments in foreign governments fixed maturity securities		3.00%	3.00%
Energy Sector \| Corporate Debt Securities
Investment [Line Items]
Investment in securities		$ 641,891,000	$ 779,720,000
Unrealized gain on investments		51,595,000	6,985,000
Europe \| Corporate Fixed Maturity and Equity Securities
Investment [Line Items]
Investment in securities		693,273,000	888,923,000
Unrealized gain on investments		$ 54,225,000	$ 67,957,000
Investment Sector Concentration Risk \| Investments \| Europe \| Financial Services Sector \| Corporate Fixed Maturity and Equity Securities
Investment [Line Items]
Percentage of European investments held in financial industry classification		23.00%	25.00%
Geographic Concentration Risk \| Investments \| United Kingdom \| Corporate Fixed Maturity and Equity Securities
Investment [Line Items]
Percentage of European investments held in financial industry classification		4.00%	5.00%
Internal Investment Grade \| Investments \| Energy Sector \| Corporate Debt Securities
Investment [Line Items]
Percentage of European investments held in financial industry classification		84.00%	89.00%
Not Hedged to U.S. Dollars \| United Kingdom, Pounds and Euro Countries, Euro \| Investment Hedging Concentration Risk \| Investments \| Europe \| Corporate Fixed Maturity and Equity Securities
Investment [Line Items]
Percentage of European investments held in financial industry classification		7.00%
Not Designated as Hedging Instrument
Investment [Line Items]
Amounts related to derivative assets		$ 25,854,000	$ 6,715,000
Amounts related to derivative liabilities		40,827,000	27,689,000
Derivative losses recorded to income statement		$ 19,297,000	$ (5,298,000)	$ (7,453,000)