Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available for Sale Amortized Cost
Amortized Cost	$ 10,145,266	$ 9,818,298
Available for Sale Securities Fair Value
Fair Value	10,110,786	9,362,763
Fixed maturities
Available for Sale Amortized Cost
Due in one year or less	253,164
Due after one year through five years	1,077,651
Due after five years through ten years	1,704,257
Due after ten years	6,102,888
Amortized Cost	10,145,266	9,818,298
Available for Sale Securities Fair Value
Due in one year or less	253,552
Due after one year through five years	1,099,278
Due after five years through ten years	1,783,306
Due after ten years	5,957,634
Fair Value	10,110,786	9,362,763
Fixed maturities \| Asset-backed securities
Available for Sale Amortized Cost
Amortized Cost	341,277	248,547
Available for Sale Securities Fair Value
Fair Value	345,587	251,309
Fixed maturities \| Commercial mortgage-backed securities
Available for Sale Amortized Cost
Amortized Cost	502,695	484,673
Available for Sale Securities Fair Value
Fair Value	505,684	484,713
Fixed maturities \| Residential mortgage-backed securities
Available for Sale Amortized Cost
Amortized Cost	163,334	196,506
Available for Sale Securities Fair Value
Fair Value	$ 165,745	$ 200,056

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the next fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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