Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Assets
Fixed maturities, available-for-sale	$ 17,293,066		$ 18,469,544
Fixed maturities held for trading	219,558		336,055
Short-term investments	3,071,133		294,287
Collateral under securities lending agreements	62,887		18,534
Collateral under derivative counterparty collateral agreements	131,303		143,710
Total derivative instruments	26,297	[1]	23,478	[1]
Separate account assets	27,481,866		26,630,904
Liabilities
Payable under securities lending agreements	62,887		18,534
Total derivative instruments	154,295	[1]	167,743	[1]
Separate account liabilities	27,481,866		26,630,904
Derivative instruments designated as hedges
Assets
Total derivative instruments	19,631	[1]	18,927	[1]
Liabilities
Total derivative instruments	4,862	[1]	7,990	[1]
Derivative instruments not designated as hedges
Assets
Total derivative instruments	6,666	[1]	4,551	[1]
Liabilities
Total derivative instruments	149,433	[1]	159,753	[1]
Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	1,840	[1]	1,454	[1]
Liabilities
Total derivative instruments	1,411	[1]	3,492	[1]
Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	424	[1]	1,176	[1]
Liabilities
Total derivative instruments	0	[1]
Derivative instruments not designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	2,055	[1]	1,921	[1]
Liabilities
Total derivative instruments	143,588	[1]	156,261	[1]
Derivative instruments not designated as hedges | Other forward contracts
Assets
Total derivative instruments	2,347	[1]
Liabilities
Total derivative instruments	4,434	[1]
U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale	901,269		3,064,639
Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale	2,164,879		1,943,587
Foreign government securities
Assets
Fixed maturities, available-for-sale	2,494		2,603
Corporate debt securities
Assets
Fixed maturities, available-for-sale	11,660,295	[2]	10,798,981	[3]
Asset-backed securities
Assets
Fixed maturities, available-for-sale	1,454,296		1,655,637
Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale	191,568		249,585
Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale	907,608		742,124
Collateralized debt obligations
Assets
Fixed maturities, available-for-sale	10,657		12,388
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Short-term investments	434,732		254,378
Collateral under securities lending agreements	62,887		18,534
Collateral under derivative counterparty collateral agreements	131,303		143,710
Separate account assets	15,583,106		14,861,680
Total assets	16,212,028		15,278,302
Liabilities
Payable under securities lending agreements	62,887		18,534
Separate account liabilities	0	[4]	2	[4]
Total liabilities	62,887		18,536
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed maturities, available-for-sale	17,077,034		18,209,902
Fixed maturities held for trading	219,558		336,055
Short-term investments	2,636,401		39,909
Total derivative instruments	26,297		23,478
Separate account assets	11,898,760		11,769,224
Total assets	31,858,050		30,378,568
Liabilities
Total derivative instruments	154,295		167,743
Separate account liabilities	80,567	[4]	166,325	[4]
Total liabilities	234,862		334,068
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges | Interest rate swaps
Assets
Total derivative instruments	18,330		18,927
Liabilities
Total derivative instruments	100		147
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	1,301
Liabilities
Total derivative instruments	4,762		7,843
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	1,840		1,454
Liabilities
Total derivative instruments	1,411		3,492
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	424		1,176
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	2,055		1,921
Liabilities
Total derivative instruments	143,588		156,261
Recurring basis | Significant other observable inputs (Level 2) | Derivative instruments not designated as hedges | Other forward contracts
Assets
Total derivative instruments	2,347
Liabilities
Total derivative instruments	4,434
Recurring basis | Significant other observable inputs (Level 2) | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale	901,269		3,064,639
Fixed maturities held for trading	160,390		236,000
Recurring basis | Significant other observable inputs (Level 2) | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale	2,164,879		1,943,587
Recurring basis | Significant other observable inputs (Level 2) | Foreign government securities
Assets
Fixed maturities, available-for-sale	2,494		2,603
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Fixed maturities, available-for-sale	11,654,190		10,792,329
Fixed maturities held for trading	58,103		58,171
Recurring basis | Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed maturities, available-for-sale	1,244,369		1,402,679
Fixed maturities held for trading			40,858
Recurring basis | Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale	191,568		249,585
Recurring basis | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale	907,608		742,124
Fixed maturities held for trading	1,065		1,026
Recurring basis | Significant other observable inputs (Level 2) | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale	10,657		12,356
Recurring basis | Fair Value, Inputs, Level 3
Assets
Fixed maturities, available-for-sale	216,032		259,642
Total assets	216,032		259,642
Recurring basis | Fair Value, Inputs, Level 3 | Corporate debt securities
Assets
Fixed maturities, available-for-sale	6,105		6,652
Recurring basis | Fair Value, Inputs, Level 3 | Asset-backed securities
Assets
Fixed maturities, available-for-sale	209,927		252,958
Recurring basis | Fair Value, Inputs, Level 3 | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale	0		32
Recurring basis | Total
Assets
Fixed maturities, available-for-sale	17,293,066		18,469,544
Fixed maturities held for trading	219,558		336,055
Short-term investments	3,071,133		294,287
Collateral under securities lending agreements	62,887		18,534
Collateral under derivative counterparty collateral agreements	131,303		143,710
Total derivative instruments	26,297		23,478
Separate account assets	27,481,866		26,630,904
Total assets	48,286,110		45,916,512
Liabilities
Payable under securities lending agreements	62,887		18,534
Total derivative instruments	154,295		167,743
Separate account liabilities	80,567	[4]	166,327	[4]
Total liabilities	297,749		352,604
Recurring basis | Total | Derivative instruments designated as hedges | Interest rate swaps
Assets
Total derivative instruments	18,330		18,927
Liabilities
Total derivative instruments	100		147
Recurring basis | Total | Derivative instruments designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	1,301
Liabilities
Total derivative instruments	4,762		7,843
Recurring basis | Total | Derivative instruments not designated as hedges | Interest rate swaps
Assets
Total derivative instruments	1,840		1,454
Liabilities
Total derivative instruments	1,411		3,492
Recurring basis | Total | Derivative instruments not designated as hedges | Interest rate swaptions
Assets
Total derivative instruments	424		1,176
Recurring basis | Total | Derivative instruments not designated as hedges | Cross-currency swaps
Assets
Total derivative instruments	2,055		1,921
Liabilities
Total derivative instruments	143,588		156,261
Recurring basis | Total | Derivative instruments not designated as hedges | Other forward contracts
Assets
Total derivative instruments	2,347
Liabilities
Total derivative instruments	4,434
Recurring basis | Total | U.S. government direct obligations and U.S. agencies
Assets
Fixed maturities, available-for-sale	901,269		3,064,639
Fixed maturities held for trading	160,390		236,000
Recurring basis | Total | Obligations of U.S. states and their subdivisions
Assets
Fixed maturities, available-for-sale	2,164,879		1,943,587
Recurring basis | Total | Foreign government securities
Assets
Fixed maturities, available-for-sale	2,494		2,603
Recurring basis | Total | Corporate debt securities
Assets
Fixed maturities, available-for-sale	11,660,295		10,798,981
Fixed maturities held for trading	58,103		58,171
Recurring basis | Total | Asset-backed securities
Assets
Fixed maturities, available-for-sale	1,454,296		1,655,637
Fixed maturities held for trading			40,858
Recurring basis | Total | Residential mortgage-backed securities
Assets
Fixed maturities, available-for-sale	191,568		249,585
Recurring basis | Total | Commercial mortgage-backed securities
Assets
Fixed maturities, available-for-sale	907,608		742,124
Fixed maturities held for trading	1,065		1,026
Recurring basis | Total | Collateralized debt obligations
Assets
Fixed maturities, available-for-sale	$ 10,657		$ 12,388

[1] 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] Includes perpetual debt investments with amortized cost of $157,742 and estimated fair value of $136,356.

[3] Includes perpetual debt investments with amortized cost of $172,054 and estimated fair value of $143,644.

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