Fair Value - Recurring by FV hierarchy (Details) - USD ($)		Dec. 31, 2016		Dec. 31, 2015
Assets
Fixed maturities, available-for-sale		$ 22,153,703,000		$ 20,531,627,000
Fixed maturities held for trading		514,738,000		615,839,000
Short-term investments		303,988,000		267,026,000
Collateral under derivative counterparty collateral agreements		0		50,924,000
Total derivative instruments	[1]	146,385,000		68,600,000
Separate account assets		27,037,765,000		26,631,193,000
Liabilities
Collateral under derivative counterparty collateral agreements		103,214,000		19,060,000
Total derivative instruments	[1]	38,281,000		76,373,000
Separate account liabilities		27,037,765,000		26,631,193,000
Derivative instruments designated as hedges:
Assets
Total derivative instruments	[1]	87,031,000		40,579,000
Liabilities
Total derivative instruments	[1]	8,294,000		22,000
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	[1]	59,354,000		28,021,000
Liabilities
Total derivative instruments	[1]	29,987,000		76,351,000
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	[1]	8,982,000		8,295,000
Liabilities
Total derivative instruments	[1]	13,340,000		5,055,000
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	[1]	50,018,000		19,537,000
Liabilities
Total derivative instruments	[1]	16,647,000		71,296,000
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	[1]	354,000		189,000
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale		3,035,112,000		3,341,752,000
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale		2,098,662,000		2,219,173,000
Foreign government securities
Assets
Fixed maturities, available-for-sale				2,286,000
Corporate debt securities
Assets
Fixed maturities, available-for-sale		13,979,749,000	[2]	12,505,712,000	[3]
Asset-backed securities
Assets
Fixed maturities, available-for-sale		1,312,379,000		1,311,370,000
Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale		140,992,000		125,372,000
Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale		1,225,282,000		1,016,908,000
Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		361,527,000		9,054,000
Recurring basis
Assets and liabilities measured at fair value
Fair value alternative investments		430,000,000		534,000,000
Assets
Fixed maturities, available-for-sale		22,153,703,000		20,531,627,000
Fixed maturities held for trading		514,738,000		615,839,000
Short-term investments		303,988,000		267,026,000
Collateral under derivative counterparty collateral agreements		103,214,000		69,984,000
Total derivative instruments		146,385,000		68,600,000
Separate account assets		27,037,765,000		26,631,193,000
Total assets		50,259,793,000		48,184,269,000
Liabilities
Collateral under derivative counterparty collateral agreements		103,214,000		19,060,000
Total derivative instruments		38,281,000		76,373,000
Embedded derivatives - GLWB		5,712,000		11,257,000
Separate account liabilities		336,523,000	[4]	290,317,000	[5]
Total liabilities		483,730,000		397,007,000
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		33,390,000		11,843,000
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		53,641,000		28,736,000
Liabilities
Total derivative instruments		8,294,000		22,000
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		8,982,000		8,295,000
Liabilities
Total derivative instruments		13,340,000		5,055,000
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		50,018,000		19,537,000
Liabilities
Total derivative instruments		16,647,000		71,296,000
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		354,000		189,000
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale		3,035,112,000		3,341,752,000
Fixed maturities held for trading		458,067,000		558,208,000
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale		2,098,662,000		2,219,173,000
Recurring basis | Foreign government securities
Assets
Fixed maturities, available-for-sale				2,286,000
Recurring basis | Corporate debt securities
Assets
Fixed maturities, available-for-sale		13,979,749,000		12,505,712,000
Fixed maturities held for trading		55,591,000		56,566,000
Recurring basis | Asset-backed securities
Assets
Fixed maturities, available-for-sale		1,312,379,000		1,311,370,000
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale		140,992,000		125,372,000
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale		1,225,282,000		1,016,908,000
Fixed maturities held for trading		1,080,000		1,065,000
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		361,527,000		9,054,000
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments		267,851,000		132,288,000
Collateral under derivative counterparty collateral agreements		103,214,000		69,984,000
Separate account assets		15,407,992,000		15,249,966,000
Total assets		15,779,057,000		15,452,238,000
Liabilities
Collateral under derivative counterparty collateral agreements		103,214,000		19,060,000
Separate account liabilities		55,000	[4]	24,000	[5]
Total liabilities		103,269,000		19,084,000
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities, available-for-sale		22,142,064,000		20,527,089,000
Fixed maturities held for trading		514,738,000		615,839,000
Short-term investments		36,137,000		134,738,000
Total derivative instruments		146,385,000		68,600,000
Separate account assets		11,199,924,000		10,847,266,000
Total assets		34,039,248,000		32,193,532,000
Liabilities
Total derivative instruments		38,281,000		76,373,000
Separate account liabilities		336,468,000	[4]	290,293,000	[5]
Total liabilities		374,749,000		366,666,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		33,390,000		11,843,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		53,641,000		28,736,000
Liabilities
Total derivative instruments		8,294,000		22,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		8,982,000		8,295,000
Liabilities
Total derivative instruments		13,340,000		5,055,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		50,018,000		19,537,000
Liabilities
Total derivative instruments		16,647,000		71,296,000
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		354,000		189,000
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale		3,035,112,000		3,341,752,000
Fixed maturities held for trading		458,067,000		558,208,000
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale		2,098,662,000		2,219,173,000
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities, available-for-sale				2,286,000
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities, available-for-sale		13,968,110,000		12,501,174,000
Fixed maturities held for trading		55,591,000		56,566,000
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities, available-for-sale		1,312,379,000		1,311,370,000
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale		140,992,000		125,372,000
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale		1,225,282,000		1,016,908,000
Fixed maturities held for trading		1,080,000		1,065,000
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		361,527,000		9,054,000
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities, available-for-sale		11,639,000		4,538,000
Total assets		11,639,000		4,538,000
Liabilities
Embedded derivatives - GLWB		5,712,000		11,257,000
Total liabilities		5,712,000		11,257,000
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities, available-for-sale		11,639,000		4,538,000
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities, available-for-sale		0		0
Recurring basis | Significant unobservable inputs (Level 3) | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		$ 0		$ 0

[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 consolidated balance sheets.

[2] Includes perpetual debt investments with amortized cost of $135,187 and estimated fair value of $113,239.

[3] Includes perpetual debt investments with amortized cost of $149,062 and estimated fair value of $116,423.

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

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