Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands		Sep. 30, 2015		Dec. 31, 2014
Assets
Fixed maturities available-for-sale:		$ 18,494,354		$ 20,162,078
Fixed maturities held-for-trading:		127,214		338,543
Short-term investments		2,463,175		263,501
Collateral under securities lending agreements		45,726		13,741
Collateral under derivative counterparty collateral agreements		66,769		106,110
Total derivative instruments		79,022	[1]	35,604	[2]
Separate account assets		26,367,569		27,718,844
Liabilities
Payable under securities lending agreements		45,726		13,741
Collateral under derivative counterparty collateral agreements		15,168		791
Total derivative instruments		107,236	[1]	136,587	[2]
Separate account liabilities		26,367,569		27,718,844
Derivative instruments designated as hedges:
Assets
Total derivative instruments		31,353	[1]	24,526	[2]
Liabilities
Total derivative instruments		983	[1]	2,952	[2]
Derivative instruments not designated as hedges:
Assets
Total derivative instruments		47,669	[1]	11,078	[2]
Liabilities
Total derivative instruments		106,253	[1]	133,635	[2]
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		10,057	[1]	6,246	[2]
Liabilities
Total derivative instruments		5,152	[1]	1,844	[2]
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		193	[1]	271	[2]
Liabilities
Total derivative instruments		0	[1]	0	[2]
Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments	[1]	23,519
Liabilities
Total derivative instruments	[1]	8,741
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		13,900	[1]	4,561	[2]
Liabilities
Total derivative instruments		92,360	[1]	131,791	[2]
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:		1,085,830		3,547,256
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:		2,288,301		2,172,484
Foreign government securities
Assets
Fixed maturities available-for-sale:		2,332		2,451
Corporate debt securities
Assets
Fixed maturities available-for-sale:		12,626,476	[3]	11,939,449	[4]
Asset-backed securities
Assets
Fixed maturities available-for-sale:		1,349,769		1,398,539
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:		136,910		173,229
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:		995,371		918,205
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:		9,365		10,465
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments		605,962		156,935
Collateral under securities lending agreements		45,726		13,741
Collateral under derivative counterparty collateral agreements		81,937		106,901
Separate account assets		14,747,770		16,146,057
Total assets		15,481,395		16,423,634
Liabilities
Payable under securities lending agreements		45,726		13,741
Collateral under derivative counterparty collateral agreements		15,168		791
Separate account liabilities	[5]	6		15
Total liabilities		60,900		14,547
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale:		18,489,531		20,156,200
Fixed maturities held-for-trading:		127,214		338,543
Short-term investments		1,857,213		106,566
Total derivative instruments		79,022		35,604
Separate account assets		11,619,799		11,572,787
Total assets		32,172,779		32,209,700
Liabilities
Total derivative instruments		107,236		136,587
Separate account liabilities	[5]	357,468		217,712
Total liabilities		464,704		354,299
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		13,978		19,383
Liabilities
Total derivative instruments				131
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		17,375		5,143
Liabilities
Total derivative instruments		983		2,821
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		10,057		6,246
Liabilities
Total derivative instruments		5,152		1,844
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		193		271
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments		23,519
Liabilities
Total derivative instruments		8,741
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		13,900		4,561
Liabilities
Total derivative instruments		92,360		131,791
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:		1,085,830		3,547,256
Fixed maturities held-for-trading:		68,853		279,602
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:		2,288,301		2,172,484
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities available-for-sale:		2,332		2,451
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:		12,621,693		11,933,607
Fixed maturities held-for-trading:		57,268		57,850
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:		1,349,729		1,398,503
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:		136,910		173,229
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:		995,371		918,205
Fixed maturities held-for-trading:		1,093		1,091
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:		9,365		10,465
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:		4,823		5,878
Total assets		4,823		5,878
Liabilities
Embedded derivatives - GLWB		9,777		6,407
Total liabilities		9,777		6,407
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale:		4,783		5,842
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale:		40		36
Recurring basis | Total
Assets
Fixed maturities available-for-sale:		18,494,354		20,162,078
Fixed maturities held-for-trading:		127,214		338,543
Short-term investments		2,463,175		263,501
Collateral under securities lending agreements		45,726		13,741
Collateral under derivative counterparty collateral agreements		81,937		106,901
Total derivative instruments		79,022		35,604
Separate account assets		26,367,569		27,718,844
Total assets		47,658,997		48,639,212
Liabilities
Payable under securities lending agreements		45,726		13,741
Collateral under derivative counterparty collateral agreements		15,168		791
Total derivative instruments		107,236		136,587
Embedded derivatives - GLWB		9,777		6,407
Separate account liabilities	[5]	357,474		217,727
Total liabilities		535,381		375,253
Recurring basis | Total | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		13,978		19,383
Liabilities
Total derivative instruments				131
Recurring basis | Total | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		17,375		5,143
Liabilities
Total derivative instruments		983		2,821
Recurring basis | Total | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		10,057		6,246
Liabilities
Total derivative instruments		5,152		1,844
Recurring basis | Total | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		193		271
Recurring basis | Total | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments		23,519
Liabilities
Total derivative instruments		8,741
Recurring basis | Total | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		13,900		4,561
Liabilities
Total derivative instruments		92,360		131,791
Recurring basis | Total | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:		1,085,830		3,547,256
Fixed maturities held-for-trading:		68,853		279,602
Recurring basis | Total | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:		2,288,301		2,172,484
Recurring basis | Total | Foreign government securities
Assets
Fixed maturities available-for-sale:		2,332		2,451
Recurring basis | Total | Corporate debt securities
Assets
Fixed maturities available-for-sale:		12,626,476		11,939,449
Fixed maturities held-for-trading:		57,268		57,850
Recurring basis | Total | Asset-backed securities
Assets
Fixed maturities available-for-sale:		1,349,769		1,398,539
Recurring basis | Total | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:		136,910		173,229
Recurring basis | Total | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:		995,371		918,205
Fixed maturities held-for-trading:		1,093		1,091
Recurring basis | Total | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:		$ 9,365		$ 10,465

[1] The estimated fair value excludes accrued income and expense. The estimated fair value of all derivatives in an asset position is reported within other assets and the estimated fair value of all derivatives in a liability position is reported within other liabilities in the condensed consolidated balance sheets.

[2] The estimated fair value excludes accrued income and expense. The estimated fair value of all derivatives in an asset position is reported within other assets and the estimated fair value of all derivatives in a liability position is reported within other liabilities in the condensed consolidated balance sheets.

[3] Includes perpetual debt investments with amortized cost of $149,062 and estimated fair value of $119,151.

[4] Includes perpetual debt investments with amortized cost of $157,742 and estimated fair value of $131,799.

[5] Includes only separate account instruments which are carried at the fair value of the underlying liabilities owned by the separate accounts