Investments (Narrative) (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) security	Dec. 31, 2014 USD ($) security	Dec. 31, 2013 USD ($)
Investment [Line Items]
Maximum individual state exposure	0.50%	0.50%
Advance refunded or escrowed-to-maturity securities	$ 319,654,000	$ 270,107,000
Percentage of revenue securities	50.00%	51.00%
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	$ 7,212,000	69,000	4,516,000
Other-than-temporary impairment recognized in earnings	5,024,000	30,000	4,387,000
Other-than-temporary impairment losses recorded as a component of AOCI	$ 2,188,000	$ 39,000	129,000
Percentage of securities representing gross unrealized losses	5.00%	2.00%
Percentage of gross unrealized losses in a continuous loss position less than twelve months	88.00%	63.00%
Individual securities comprising total gross unrealized losses (in shares) | security	884	385
Percentage of residential mortgage-backed holdings exposure to sub-prime mortgage collateral	2.00%
Percentage of fixed income portfolio represents security with sub-prime exposure	1.00%
Percentage of unrealized gain position represents security with sub-prime exposure	2.00%
Percentage of security with sub-prime exposure rated as investment grade	9.00%
Approximate percentage, outstanding principal balance of commercial mortgage loans	41.00%
Outstanding balance of commercial mortgage loans	$ 1,153,838,000	$ 1,276,015,000
Commercial mortgage loan valuation allowance	2,582,000	3,399,000
Loan valuation allowance decrease	817,000	(1,083,000)
Mortgage loan commitments outstanding	6,350,000
Committed to fund additional capital contributions to joint ventures	28,607,000
Short term investments and fixed maturities	441,851,000	519,659,000
Other assets	475,731,000	847,860,000
(Loss) gain on investments	(31,826,000)	(60,783,000)	(34,525,000)
Maximum exposure to loss related to VIEs	56,781,000
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 28,607,000
Percentage of securities received as collateral	102.00%
Collateral held under securities lending	$ 0	95,985,000
Liability to the borrower for collateral	0	95,986,000
Minimum
Investment [Line Items]
Outstanding balance of commercial mortgage loans	17,000	77,000
Maximum
Investment [Line Items]
Outstanding balance of commercial mortgage loans	$ 14,625,000	$ 15,190,000
Canadian Government/Provincials
Investment [Line Items]
Percentage of investments in foreign governments fixed maturity securities	79.00%	76.00%
Governments Of Brazil
Investment [Line Items]
Percentage of investments in foreign governments fixed maturity securities	8.00%	10.00%
Governments Of Germany
Investment [Line Items]
Percentage of investments in foreign governments fixed maturity securities	5.00%	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	$ 779,720,000	$ 992,012,000
Unrealized gain on investments	6,985,000	89,590,000
Europe | Corporate Fixed Maturity and Equity Securities
Investment [Line Items]
Investment in securities	888,923,000	1,060,655,000
Unrealized gain on investments	$ 67,957,000	$ 116,975,000
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	25.00%	22.00%
Geographic Concentration Risk | Investments | Unspecified European Country | Corporate Fixed Maturity and Equity Securities
Investment [Line Items]
Percentage of European investments held in financial industry classification	5.00%	5.00%
Internal Investment Grade | Investments | Energy Sector | Corporate Debt Securities
Investment [Line Items]
Percentage of European investments held in financial industry classification	89.00%	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	6.00%
Not Designated as Hedging Instrument [Member]
Investment [Line Items]
Amounts related to derivative assets	$ 6,715,000	$ 9,040,000
Amounts related to derivative liabilities	27,689,000	25,303,000
Derivative losses recorded to income statement	$ 5,298,000	$ 7,453,000	$ 703,000