Fair Value Measurements (Securities in Unrealized Loss Positions) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	$ (1,187)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	485,129
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(85)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	72,550
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(1,272)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	557,679
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(292)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	37,802
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	9,994
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(298)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	47,796
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(51)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	17,582
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(51)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	17,582
Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(844)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	429,745
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(79)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	62,556
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(923)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 492,301