Fair Value Measurements - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands		Jun. 30, 2015		Dec. 31, 2014
Assets
Fixed maturities available-for-sale:		$ 17,977,077		$ 20,162,078
Fixed maturities held-for-trading:		130,577		338,543
Short-term investments		2,635,110		263,501
Collateral under securities lending agreements		88,660		13,741
Collateral under derivative counterparty collateral agreements		120,107		106,110
Total derivative instruments		44,646	[1]	35,604	[2]
Separate account assets		27,505,755		27,718,844
Liabilities
Payable under securities lending agreements		88,660		13,741
Collateral under derivative counterparty collateral agreements		820		791
Total derivative instruments		162,461	[1]	136,587	[2]
Separate account liabilities		27,505,755		27,718,844
Derivative instruments designated as hedges:
Assets
Total derivative instruments		25,570	[1]	24,526	[2]
Liabilities
Total derivative instruments		7,577	[1]	2,952	[2]
Derivative instruments not designated as hedges:
Assets
Total derivative instruments		19,076	[1]	11,078	[2]
Liabilities
Total derivative instruments		154,884	[1]	133,635	[2]
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		4,620	[1]	6,246	[2]
Liabilities
Total derivative instruments		6,179	[1]	1,844	[2]
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		260	[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]	7,157
Liabilities
Total derivative instruments	[1]	24,917
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		7,039	[1]	4,561	[2]
Liabilities
Total derivative instruments		123,788	[1]	131,791	[2]
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:		927,698		3,547,256
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:		2,311,904		2,172,484
Foreign government securities
Assets
Fixed maturities available-for-sale:		2,381		2,451
Corporate debt securities
Assets
Fixed maturities available-for-sale:		12,251,090	[3]	11,939,449	[4]
Asset-backed securities
Assets
Fixed maturities available-for-sale:		1,372,834		1,398,539
Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:		147,188		173,229
Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:		954,447		918,205
Collateralized debt obligations
Assets
Fixed maturities available-for-sale:		9,535		10,465
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments		193,146		156,935
Collateral under securities lending agreements		88,660		13,741
Collateral under derivative counterparty collateral agreements		120,927		106,901
Separate account assets		16,103,914		16,146,057
Total assets		16,506,647		16,423,634
Liabilities
Payable under securities lending agreements		88,660		13,741
Collateral under derivative counterparty collateral agreements		820		791
Separate account liabilities	[5]	7		15
Total liabilities		89,487		14,547
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities available-for-sale:		17,971,981		20,156,200
Fixed maturities held-for-trading:		130,577		338,543
Short-term investments		2,441,964		106,566
Total derivative instruments		44,646		35,604
Separate account assets		11,401,841		11,572,787
Total assets		31,991,009		32,209,700
Liabilities
Total derivative instruments		162,461		136,587
Separate account liabilities	[5]	272,031		217,712
Total liabilities		434,492		354,299
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		16,458		19,383
Liabilities
Total derivative instruments		9		131
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		9,112		5,143
Liabilities
Total derivative instruments		7,568		2,821
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		4,620		6,246
Liabilities
Total derivative instruments		6,179		1,844
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		260		271
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments		7,157
Liabilities
Total derivative instruments		24,917
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		7,039		4,561
Liabilities
Total derivative instruments		123,788		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:		927,698		3,547,256
Fixed maturities held-for-trading:		72,118		279,602
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:		2,311,904		2,172,484
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities available-for-sale:		2,381		2,451
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities available-for-sale:		12,246,025		11,933,607
Fixed maturities held-for-trading:		57,384		57,850
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities available-for-sale:		1,372,803		1,398,503
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:		147,188		173,229
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:		954,447		918,205
Fixed maturities held-for-trading:		1,075		1,091
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:		9,535		10,465
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities available-for-sale:		5,096		5,878
Total assets		5,096		5,878
Liabilities
Embedded derivatives - GLWB		2,919		6,407
Total liabilities		2,919		6,407
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities available-for-sale:		5,065		5,842
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities available-for-sale:		31		36
Recurring basis | Total
Assets
Fixed maturities available-for-sale:		17,977,077		20,162,078
Fixed maturities held-for-trading:		130,577		338,543
Short-term investments		2,635,110		263,501
Collateral under securities lending agreements		88,660		13,741
Collateral under derivative counterparty collateral agreements		120,927		106,901
Total derivative instruments		44,646		35,604
Separate account assets		27,505,755		27,718,844
Total assets		48,502,752		48,639,212
Liabilities
Payable under securities lending agreements		88,660		13,741
Collateral under derivative counterparty collateral agreements		820		791
Total derivative instruments		162,461		136,587
Embedded derivatives - GLWB		2,919		6,407
Separate account liabilities	[5]	272,038		217,727
Total liabilities		526,898		375,253
Recurring basis | Total | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		16,458		19,383
Liabilities
Total derivative instruments		9		131
Recurring basis | Total | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		9,112		5,143
Liabilities
Total derivative instruments		7,568		2,821
Recurring basis | Total | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		4,620		6,246
Liabilities
Total derivative instruments		6,179		1,844
Recurring basis | Total | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		260		271
Recurring basis | Total | Derivative instruments not designated as hedges: | Other forward contracts
Assets
Total derivative instruments		7,157
Liabilities
Total derivative instruments		24,917
Recurring basis | Total | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		7,039		4,561
Liabilities
Total derivative instruments		123,788		131,791
Recurring basis | Total | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities available-for-sale:		927,698		3,547,256
Fixed maturities held-for-trading:		72,118		279,602
Recurring basis | Total | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities available-for-sale:		2,311,904		2,172,484
Recurring basis | Total | Foreign government securities
Assets
Fixed maturities available-for-sale:		2,381		2,451
Recurring basis | Total | Corporate debt securities
Assets
Fixed maturities available-for-sale:		12,251,090		11,939,449
Fixed maturities held-for-trading:		57,384		57,850
Recurring basis | Total | Asset-backed securities
Assets
Fixed maturities available-for-sale:		1,372,834		1,398,539
Recurring basis | Total | Residential mortgage-backed securities
Assets
Fixed maturities available-for-sale:		147,188		173,229
Recurring basis | Total | Commercial mortgage-backed securities
Assets
Fixed maturities available-for-sale:		954,447		918,205
Fixed maturities held-for-trading:		1,075		1,091
Recurring basis | Total | Collateralized debt obligations
Assets
Fixed maturities available-for-sale:		$ 9,535		$ 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 $157,742 and estimated fair value of $129,537.

[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