Summary of Investments (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Securities available-for-sale
Amortized cost	$ 16,182,197		$ 17,807,359
Estimated fair value and carrying value	17,293,066		18,469,544
Fixed maturities
Securities available-for-sale
Amortized cost	16,182,197		17,807,359
Gross unrealized gains	1,221,545		970,407
Gross unrealized losses	110,676		308,222
Estimated fair value and carrying value	17,293,066		18,469,544
OTTI (gain) loss included in AOCI	(93,377)	[1]	(101,184)	[1]
U.S. government direct obligations and U.S. agencies
Securities available-for-sale
Amortized cost	845,069		3,044,185
Gross unrealized gains	59,509		43,827
Gross unrealized losses	3,309		23,373
Estimated fair value and carrying value	901,269		3,064,639
OTTI (gain) loss included in AOCI	0	[1]	0	[1]
Obligations of U.S. states and their subdivisions
Securities available-for-sale
Amortized cost	1,902,663		1,763,797
Gross unrealized gains	264,962		196,742
Gross unrealized losses	2,746		16,952
Estimated fair value and carrying value	2,164,879		1,943,587
OTTI (gain) loss included in AOCI	0	[1]	0	[1]
Foreign government securities
Securities available-for-sale
Amortized cost	2,496		2,617
Gross unrealized gains	0		0
Gross unrealized losses	2		14
Estimated fair value and carrying value	2,494		2,603
OTTI (gain) loss included in AOCI	0	[1]	0	[1]
Corporate debt securities
Securities available-for-sale
Amortized cost	11,013,781	[2]	10,454,252	[3]
Gross unrealized gains	729,468	[2]	568,261	[3]
Gross unrealized losses	82,954	[2]	223,532	[3]
Estimated fair value and carrying value	11,660,295	[2]	10,798,981	[3]
OTTI (gain) loss included in AOCI	(2,299)	[1],[2]	(2,553)	[1],[3]
Perpetual debt investments
Securities available-for-sale
Amortized cost	157,742		172,054
Estimated fair value and carrying value	136,356		143,644
Asset-backed securities
Securities available-for-sale
Amortized cost	1,334,543		1,553,510
Gross unrealized gains	134,218		131,277
Gross unrealized losses	14,465		29,150
Estimated fair value and carrying value	1,454,296		1,655,637
OTTI (gain) loss included in AOCI	(90,795)	[1]	(98,502)	[1]
Residential mortgage-backed securities
Securities available-for-sale
Amortized cost	186,087		244,723
Gross unrealized gains	8,277		8,335
Gross unrealized losses	2,796		3,473
Estimated fair value and carrying value	191,568		249,585
OTTI (gain) loss included in AOCI	(283)	[1]	(129)	[1]
Commercial mortgage-backed securities
Securities available-for-sale
Amortized cost	886,633		731,688
Gross unrealized gains	25,111		21,951
Gross unrealized losses	4,136		11,515
Estimated fair value and carrying value	907,608		742,124
OTTI (gain) loss included in AOCI	0	[1]	0	[1]
Collateralized debt obligations
Securities available-for-sale
Amortized cost	10,925		12,587
Gross unrealized gains	0		14
Gross unrealized losses	268		213
Estimated fair value and carrying value	10,657		12,388
OTTI (gain) loss included in AOCI	$ 0	[1]	$ 0	[1]

[1] Indicates the amount of any OTTI (gain) loss included in AOCI that is included in gross unrealized gains and losses. OTTI (gain) loss included in AOCI, as presented above, includes both the initial recognition of non-credit losses and the effects of subsequent increases and decreases in estimated fair value for those fixed maturity securities with previous non-credit impairment. The non-credit loss component of OTTI (gain) loss was in an unrealized gain position due to increases in estimated fair value subsequent to initial recognition of non-credit losses on such securities.

[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.