Fair Value Measurements (Securities in Unrealized Loss Positions) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Jun. 30, 2014	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	$ (3,894)	$ (83,095)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	256,130	2,065,491
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(55,215)	(1,711)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,661,503	190,357
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(59,109)	(84,806)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,917,633	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	(98)	(660)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	24,903	52,434
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(62)	(238)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	9,938	9,763
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(160)	(898)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	34,841	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	(464)	(4,144)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	74,536	309,109
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(602)	(66)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	217,016	14,091
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(1,066)	(4,210)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	291,552	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	(3,332)	(78,291)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	156,691	1,703,948
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	(54,551)	(1,407)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,434,549	166,503
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	(57,883)	(79,698)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 1,591,240	$ 1,870,451