Investments - Fair Value and Unrealized Losses (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 securities	Dec. 31, 2012 securities
Fair value
Less than 12 months, Fair Value	$ 982,363	$ 253,333
12 Months or More, Fair value	162,774	143,853
Total, Fair value	1,145,137	397,186
Unrealized losses
Less than 12 months, Unrealized losses	(45,392)	(2,571)
12 Months or More, Unrealized losses	(3,152)	(5,445)
Total, Unrealized losses	(48,544)	(8,016)
Number of securities in loss position (in securities)	109	32
12 Months or More, number of securities (in securities)	11	9
U.S. agency bonds – mortgage-backed [Member]
Fair value
Less than 12 Months, Fair value	601,779	158,591
12 Months or More, Fair value	39,548	0
Total, Fair value	641,327	158,591
Unrealized losses
Less than 12 Months, Unrealized losses	(23,199)	(1,473)
12 Months or More, Unrealized losses	(1,031)	0
Total, Unrealized losses	(24,230)	(1,473)
Non-U.S. government bonds [Member]
Fair value
Less than 12 Months, Fair value	10,234
12 Months or More, Fair value	0
Total, Fair value	10,234
Unrealized losses
Less than 12 Months, Unrealized losses	(444)
12 Months or More, Unrealized losses	0
Total, Unrealized losses	(444)
Other mortgage-backed bonds [Member]
Fair value
Less than 12 Months, Fair value	13,152
12 Months or More, Fair value	0
Total, Fair value	13,152
Unrealized losses
Less than 12 Months, Unrealized losses	(59)
12 Months or More, Unrealized losses	0
Total, Unrealized losses	(59)
Corporate bonds [Member]
Fair value
Less than 12 Months, Fair value	354,698	94,742
12 Months or More, Fair value	123,226	141,842
Total, Fair value	477,924	236,584
Unrealized losses
Less than 12 Months, Unrealized losses	(21,689)	(1,098)
12 Months or More, Unrealized losses	(2,121)	(5,394)
Total, Unrealized losses	(23,810)	(6,492)
Municipal bonds [Member]
Fair value
Less than 12 Months, Fair value	2,500
12 Months or More, Fair value	0
Total, Fair value	2,500
Unrealized losses
Less than 12 Months, Unrealized losses	(1)
12 Months or More, Unrealized losses	0
Total, Unrealized losses	(1)
Available-for-sale securities [Member]
Fair value
Less than 12 Months, Fair value		253,333
12 Months or More, Fair value		141,842
Total, Fair value		395,175
Unrealized losses
Less than 12 Months, Unrealized losses		(2,571)
12 Months or More, Unrealized losses		(5,394)
Total, Unrealized losses		(7,965)
Other investments [Member]
Fair value
Less than 12 months, Fair Value		0
12 Months or More, Fair value		2,011
Total, Fair value		2,011
Unrealized losses
Less than 12 months, Unrealized losses		0
12 Months or More, Unrealized losses		(51)
Total, Unrealized losses		$ (51)