Available-for-sale securities Summary of available-for-sale debt securities in a continuous unrealized loss position deemed not to be other-than-temporarily impaired (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	$ 261	$ 30,528
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	139,436	1,269,712
Continuous Unrealized Loss Position, 12 Months or Greater, Gross Unrealized Losses	0	4,548
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0	46,952
Continuous Unrealized Loss Position, Gross Unrealized Losses	261	35,076
Continuous Unrealized Loss Position, Estimated Fair Value	139,436	1,316,664
US treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	0	0
Continuous Unrealized Loss Position, 12 Months or Greater, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0	0
Continuous Unrealized Loss Position, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, Estimated Fair Value	0	0
US Government Agencies Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	29	0
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	26,306	0
Continuous Unrealized Loss Position, 12 Months or Greater, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0	0
Continuous Unrealized Loss Position, Gross Unrealized Losses	29	0
Continuous Unrealized Loss Position, Estimated Fair Value	26,306	0
Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	7	0
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	3,993	0
Continuous Unrealized Loss Position, 12 Months or Greater, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0	0
Continuous Unrealized Loss Position, Gross Unrealized Losses	7	0
Continuous Unrealized Loss Position, Estimated Fair Value	3,993	0
Non-U.S. government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	0	30,505
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	0	24,741
Continuous Unrealized Loss Position, 12 Months or Greater, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0	0
Continuous Unrealized Loss Position, Gross Unrealized Losses	0	30,505
Continuous Unrealized Loss Position, Estimated Fair Value	0	24,741
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	84	5
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	72,722	224,989
Continuous Unrealized Loss Position, 12 Months or Greater, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0	0
Continuous Unrealized Loss Position, Gross Unrealized Losses	84	5
Continuous Unrealized Loss Position, Estimated Fair Value	72,722	224,989
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	0	18
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	0	1,019,982
Continuous Unrealized Loss Position, 12 Months or Greater, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0	0
Continuous Unrealized Loss Position, Gross Unrealized Losses	0	18
Continuous Unrealized Loss Position, Estimated Fair Value	0	1,019,982
Residential mortgage-backed and asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	141	0
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	36,415	0
Continuous Unrealized Loss Position, 12 Months or Greater, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0	0
Continuous Unrealized Loss Position, Gross Unrealized Losses	141	0
Continuous Unrealized Loss Position, Estimated Fair Value	36,415	0
Student loan-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	0	0
Continuous Unrealized Loss Position, Less than 12 Months, Estimated Fair Value	0	0
Continuous Unrealized Loss Position, 12 Months or Greater, Gross Unrealized Losses	0	4,548
Continuous Unrealized Loss Position, 12 Months or Greater, Estimated Fair Value	0	46,952
Continuous Unrealized Loss Position, Gross Unrealized Losses	0	4,548
Continuous Unrealized Loss Position, Estimated Fair Value	$ 0	$ 46,952