Fair Value - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Assets
Fixed maturities, available-for-sale		$ 23,593,139		$ 22,153,703
Fixed maturities held for trading		21,059		514,738
Short-term investments		350,266		303,988
Collateral under derivative counterparty collateral agreements				0
Total derivative instruments	[1]	51,359		146,385
Separate account assets		27,660,571		27,037,765
Liabilities
Collateral under derivative counterparty collateral agreements		5,490		103,214
Total derivative instruments	[1]	94,543		38,281
Separate account liabilities		27,660,571		27,037,765
Derivative instruments designated as hedges:
Assets
Total derivative instruments	[1]	27,434		87,031
Liabilities
Total derivative instruments	[1]	51,316		8,294
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	[1]	23,925		59,354
Liabilities
Total derivative instruments	[1]	43,227		29,987
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	[1]	3,530		8,982
Liabilities
Total derivative instruments	[1]	1,628		13,340
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	[1]	20,320		50,018
Liabilities
Total derivative instruments	[1]	41,599		16,647
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	[1]	75		354
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale		1,871,642		3,035,112
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale		2,090,972		2,098,662
Corporate debt securities
Assets
Fixed maturities, available-for-sale		15,706,087	[2]	13,979,749	[3]
Asset-backed securities
Assets
Fixed maturities, available-for-sale		1,717,976		1,312,379
Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale		64,984		140,992
Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale		1,357,609		1,225,282
Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		783,869		361,527
Recurring basis
Assets
Fixed maturities, available-for-sale		23,593,139		22,153,703
Fixed maturities held for trading		21,059		514,738
Short-term investments		350,266		303,988
Collateral under derivative counterparty collateral agreements		57,820		103,214
Total derivative instruments		51,359		146,385
Separate account assets		27,660,571		27,037,765
Total assets		51,734,214		50,259,793
Liabilities
Collateral under derivative counterparty collateral agreements		5,490		103,214
Total derivative instruments		94,543		38,281
Embedded derivatives - GLWB		11,095		5,712
Separate account liabilities		409,274	[4]	336,523	[5]
Total liabilities		520,402		483,730
Fair value alternative investments		400,000		430,000
Recurring basis | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		7,476		33,390
Recurring basis | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		19,958		53,641
Liabilities
Total derivative instruments		51,316		8,294
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		3,530		8,982
Liabilities
Total derivative instruments		1,628		13,340
Recurring basis | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		20,320		50,018
Liabilities
Total derivative instruments		41,599		16,647
Recurring basis | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		75		354
Recurring basis | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale		1,871,642		3,035,112
Fixed maturities held for trading		16,836		458,067
Recurring basis | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale		2,090,972		2,098,662
Recurring basis | Corporate debt securities
Assets
Fixed maturities, available-for-sale		15,706,087		13,979,749
Fixed maturities held for trading		3,156		55,591
Recurring basis | Asset-backed securities
Assets
Fixed maturities, available-for-sale		1,717,976		1,312,379
Recurring basis | Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale		64,984		140,992
Recurring basis | Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale		1,357,609		1,225,282
Fixed maturities held for trading		1,067		1,080
Recurring basis | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		783,869		361,527
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments		288,302		267,851
Collateral under derivative counterparty collateral agreements		57,820		103,214
Separate account assets		16,523,630		15,407,992
Total assets		16,869,752		15,779,057
Liabilities
Collateral under derivative counterparty collateral agreements		5,490		103,214
Separate account liabilities		8	[4]	55	[5]
Total liabilities		5,498		103,269
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities, available-for-sale		23,583,401		22,142,064
Fixed maturities held for trading		21,059		514,738
Short-term investments		61,964		36,137
Collateral under derivative counterparty collateral agreements		0
Total derivative instruments		51,359		146,385
Separate account assets		10,736,532		11,199,924
Total assets		34,454,315		34,039,248
Liabilities
Total derivative instruments		94,543		38,281
Separate account liabilities		409,266	[4]	336,468	[5]
Total liabilities		503,809		374,749
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		7,476		33,390
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		19,958		53,641
Liabilities
Total derivative instruments		51,316		8,294
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		3,530		8,982
Liabilities
Total derivative instruments		1,628		13,340
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		20,320		50,018
Liabilities
Total derivative instruments		41,599		16,647
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		75		354
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale		1,871,642		3,035,112
Fixed maturities held for trading		16,836		458,067
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale		2,090,972		2,098,662
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities, available-for-sale		15,696,349		13,968,110
Fixed maturities held for trading		3,156		55,591
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities, available-for-sale		1,717,976		1,312,379
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale		64,984		140,992
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale		1,357,609		1,225,282
Fixed maturities held for trading		1,067		1,080
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		783,869		361,527
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities, available-for-sale		9,738		11,639
Total assets		9,738		11,639
Liabilities
Embedded derivatives - GLWB		11,095		5,712
Total liabilities		11,095		5,712
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities, available-for-sale		9,738		11,639
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 $89,267 and estimated fair value of $87,348.

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

[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.