Fair Value - Recurring by FV hierarchy (Details) - USD ($) $ in Thousands		Dec. 31, 2015		Dec. 31, 2014
Assets
Fixed maturities, available-for-sale		$ 20,531,627		$ 20,162,078
Fixed maturities held for trading		615,839		338,543
Short-term investments		267,026		263,501
Collateral under securities lending agreements		0		13,741
Collateral under derivative counterparty collateral agreements		50,924		106,110
Total derivative instruments	[1]	68,600		35,604
Separate account assets		26,631,193		27,718,844
Liabilities
Payable under securities lending agreements		0		13,741
Collateral under derivative counterparty collateral agreements		19,060		791
Total derivative instruments	[1]	76,373		136,587
Separate account liabilities		26,631,193		27,718,844
Derivative instruments designated as hedges:
Assets
Total derivative instruments	[1]	40,579		24,526
Liabilities
Total derivative instruments	[1]	22		2,952
Derivative instruments not designated as hedges:
Assets
Total derivative instruments	[1]	28,021		11,078
Liabilities
Total derivative instruments	[1]	76,351		133,635
Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments	[1]	8,295		6,246
Liabilities
Total derivative instruments	[1]	5,055		1,844
Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments	[1]	189		271
Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments	[1]	19,537		4,561
Liabilities
Total derivative instruments	[1]	71,296		131,791
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale		3,341,752		3,547,256
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale		2,219,173		2,172,484
Foreign government securities
Assets
Fixed maturities, available-for-sale		2,286		2,451
Corporate debt securities
Assets
Fixed maturities, available-for-sale		12,505,712	[2]	11,939,449	[3]
Asset-backed securities
Assets
Fixed maturities, available-for-sale		1,311,370		1,398,539
Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale		125,372		173,229
Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale		1,016,908		918,205
Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		9,054		10,465
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments		132,288		156,935
Collateral under securities lending agreements				13,741
Collateral under derivative counterparty collateral agreements		69,984		106,901
Separate account assets		15,249,966		16,146,057
Total assets		15,452,238		16,423,634
Liabilities
Payable under securities lending agreements				13,741
Collateral under derivative counterparty collateral agreements		19,060		791
Separate account liabilities		24	[4]	15	[5]
Total liabilities		19,084		14,547
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities, available-for-sale		20,527,089		20,156,200
Fixed maturities held for trading		615,839		338,543
Short-term investments		134,738		106,566
Total derivative instruments		68,600		35,604
Separate account assets		11,381,227		11,572,787
Total assets		32,727,493		32,209,700
Liabilities
Total derivative instruments		76,373		136,587
Separate account liabilities		290,293	[4]	217,712	[5]
Total liabilities		366,666		354,299
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		11,843		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		28,736		5,143
Liabilities
Total derivative instruments		22		2,821
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		8,295		6,246
Liabilities
Total derivative instruments		5,055		1,844
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		189		271
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		19,537		4,561
Liabilities
Total derivative instruments		71,296		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		3,341,752		3,547,256
Fixed maturities held for trading		558,208		279,602
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale		2,219,173		2,172,484
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities, available-for-sale		2,286		2,451
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities, available-for-sale		12,501,174		11,933,607
Fixed maturities held for trading		56,566		57,850
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities, available-for-sale		1,311,370		1,398,503
Fixed maturities held for trading				0
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale		125,372		173,229
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale		1,016,908		918,205
Fixed maturities held for trading		1,065		1,091
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		9,054		10,465
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed maturities, available-for-sale		4,538		5,878
Total assets		4,538		5,878
Liabilities
Embedded derivatives - GLWB		11,257
Total liabilities		11,257
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Fixed maturities, available-for-sale		4,538		5,842
Recurring basis | Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed maturities, available-for-sale		0		36
Recurring basis | Significant unobservable inputs (Level 3) | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		0		0
Recurring basis | Total
Assets
Fixed maturities, available-for-sale		20,531,627		20,162,078
Fixed maturities held for trading		615,839		338,543
Short-term investments		267,026		263,501
Collateral under securities lending agreements				13,741
Collateral under derivative counterparty collateral agreements		69,984		106,901
Total derivative instruments		68,600		35,604
Separate account assets		26,631,193		27,718,844
Total assets		48,184,269		48,639,212
Liabilities
Payable under securities lending agreements				13,741
Collateral under derivative counterparty collateral agreements		19,060		791
Total derivative instruments		76,373		136,587
Embedded derivatives - GLWB		11,257
Separate account liabilities		290,317	[4]	217,727	[5]
Total liabilities		397,007		368,846
Recurring basis | Total | Derivative instruments designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		11,843		19,383
Liabilities
Total derivative instruments				131
Recurring basis | Total | Derivative instruments designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		28,736		5,143
Liabilities
Total derivative instruments		22		2,821
Recurring basis | Total | Derivative instruments not designated as hedges: | Interest rate swaps
Assets
Total derivative instruments		8,295		6,246
Liabilities
Total derivative instruments		5,055		1,844
Recurring basis | Total | Derivative instruments not designated as hedges: | Interest rate swaptions
Assets
Total derivative instruments		189		271
Recurring basis | Total | Derivative instruments not designated as hedges: | Cross-currency swaps
Assets
Total derivative instruments		19,537		4,561
Liabilities
Total derivative instruments		71,296		131,791
Recurring basis | Total | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale		3,341,752		3,547,256
Fixed maturities held for trading		558,208		279,602
Recurring basis | Total | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale		2,219,173		2,172,484
Recurring basis | Total | Foreign government securities
Assets
Fixed maturities, available-for-sale		2,286		2,451
Recurring basis | Total | Corporate debt securities
Assets
Fixed maturities, available-for-sale		12,505,712		11,939,449
Fixed maturities held for trading		56,566		57,850
Recurring basis | Total | Asset-backed securities
Assets
Fixed maturities, available-for-sale		1,311,370		1,398,539
Fixed maturities held for trading				0
Recurring basis | Total | Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale		125,372		173,229
Recurring basis | Total | Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale		1,016,908		918,205
Fixed maturities held for trading		1,065		1,091
Recurring basis | Total | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale		$ 9,054		$ 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 consolidated balance sheets.

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

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

[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.