Fair Value Measurements - Securities in Unrealized Loss Positions (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross loss	$ (1,249)	$ (812)
Less than 12 Months, fair value	381,422	360,665
12 months or longer, aggregate losses	(24,124)	(57,353)
12 months or longer, fair value	1,456,446	1,642,718
Aggregate losses	(25,373)	(58,165)
Fair value	1,837,868	2,003,383
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross loss	(406)	(125)
Less than 12 Months, fair value	72,688	24,875
12 months or longer, aggregate losses	(897)	(845)
12 months or longer, fair value	24,104	24,155
Aggregate losses	(1,303)	(970)
Fair value	96,792	49,030
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross loss	(422)	(472)
Less than 12 Months, fair value	246,045	316,578
12 months or longer, aggregate losses	(436)	(797)
12 months or longer, fair value	99,564	109,354
Aggregate losses	(858)	(1,269)
Fair value	345,609	425,932
Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross loss	(421)	(215)
Less than 12 Months, fair value	62,689	19,212
12 months or longer, aggregate losses	(22,791)	(55,711)
12 months or longer, fair value	1,332,778	1,509,209
Aggregate losses	(23,212)	(55,926)
Fair value	$ 1,395,467	$ 1,528,421

Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

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Amount of unrealized loss on securities classified as available-for-sale.

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

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