Fair Value Measurements (Securities in Unrealized Loss Positions) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	$ (108,409)	$ (83,095)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,788,706	2,065,491
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(16,667)	(1,711)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	434,375	190,357
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(125,076)	(84,806)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	3,223,081	2,255,848
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	(522)	(660)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	54,478	52,434
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(312)	(238)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	32,782	9,763
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(834)	(898)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	87,260	62,197
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	(5,819)	(4,144)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	523,034	309,109
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(70)	(66)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	13,067	14,091
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(5,889)	(4,210)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	536,101	323,200
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	(102,068)	(78,291)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,211,194	1,703,948
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(16,285)	(1,407)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	388,526	166,503
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(118,353)	(79,698)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 2,599,720	$ 1,870,451