Investments (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position
Less than Twelve Months, Fair Value	$ 526,576	$ 635,964
Less than 12 Months, Gross Unrealized Loss	9,545	30,309
Twelve Months of Longer, Fair Value	467,620	516,538
12 Months or Longer, Gross Unrealized Loss	94,919	114,011
Total, Fair Value	994,196	1,152,502
Total, Gross Unrealized Loss	104,464	144,320
Total, Gross Unrealized Loss	2	1
Less than Twelve Months, Fair Value	532,861	641,737
Less than 12 Months, Gross Unrealized Loss	9,547	30,310
Twelve Months or Longer, Fair Value	467,620	516,538
12 Months or Longer, Gross Unrealized Losses	94,919	114,011
Total, Fair Value	1,000,481	1,158,275
Total, Gross Unrealized Loss	104,466	144,321
Municipal obligations [Member]
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position
Less than Twelve Months, Fair Value	8,205	9,359
Less than 12 Months, Gross Unrealized Loss	6	309
Twelve Months of Longer, Fair Value	7,333	14,635
12 Months or Longer, Gross Unrealized Loss	74	174
Total, Fair Value	15,538	23,994
Total, Gross Unrealized Loss	80	483
Corporate obligations [Member]
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position
Less than Twelve Months, Fair Value	47,050	155,528
Less than 12 Months, Gross Unrealized Loss	1,122	6,220
Twelve Months of Longer, Fair Value	143,704	178,861
12 Months or Longer, Gross Unrealized Loss	12,647	16,983
Total, Fair Value	190,754	334,389
Total, Gross Unrealized Loss	13,769	23,203
U.S. government obligations [Member]
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position
Less than Twelve Months, Fair Value	297,450	130,422
Less than 12 Months, Gross Unrealized Loss	297	126
Total, Fair Value	297,450	130,422
Total, Gross Unrealized Loss	297	126
Residential Mortgage-backed Securities [Member]
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position
Less than Twelve Months, Fair Value	136,572	125,826
Less than 12 Months, Gross Unrealized Loss	7,919	14,495
Twelve Months of Longer, Fair Value	111,500	105,705
12 Months or Longer, Gross Unrealized Loss	32,568	43,321
Total, Fair Value	248,072	231,531
Total, Gross Unrealized Loss	40,487	57,816
Collateralized debt obligations [Member]
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position
Less than Twelve Months, Fair Value	18,279	33,037
Less than 12 Months, Gross Unrealized Loss	197	840
Twelve Months of Longer, Fair Value	9,558	12,482
12 Months or Longer, Gross Unrealized Loss	1,017	1,751
Total, Fair Value	27,837	45,519
Total, Gross Unrealized Loss	1,214	2,591
Other asset-backed securities [Member]
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position
Less than Twelve Months, Fair Value	19,020	181,792
Less than 12 Months, Gross Unrealized Loss	4	8,319
Twelve Months of Longer, Fair Value	195,525	204,855
12 Months or Longer, Gross Unrealized Loss	48,613	51,782
Total, Fair Value	214,545	386,647
Total, Gross Unrealized Loss	48,617	60,101
Short-term Investments [Member]
Schedule of Available for Sale Securities in a Continuous Unrealized Loss Position
Less than Twelve Months, Fair Value	6,285	5,773
Less than 12 Months, Gross Unrealized Loss	2	1
Total, Fair Value	6,285	5,773
Total, Gross Unrealized Loss	$ 2	$ 1