Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	$ (8,785)	$ (18,442)
Less than 12 months, fair value	587,525	1,187,758
12 months or more, unrealized gross losses	(52,337)	(3,851)
12 months or more, fair value	1,690,246	394,922
Total unrealized gross losses	(61,122)	(22,293)
Total, fair value	2,277,771	1,582,680
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	0	0
Less than 12 months, fair value	0	0
12 months or more, unrealized gross losses	(2,044)	(577)
12 months or more, fair value	97,956	49,423
Total unrealized gross losses	(2,044)	(577)
Total, fair value	97,956	49,423
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(12)
Less than 12 months, fair value	3,729
12 months or more, unrealized gross losses	0
12 months or more, fair value	0
Total unrealized gross losses	(12)
Total, fair value	3,729
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	0	(759)
Less than 12 months, fair value	0	24,400
12 months or more, unrealized gross losses	(1,851)	(697)
12 months or more, fair value	164,140	96,195
Total unrealized gross losses	(1,851)	(1,456)
Total, fair value	164,140	120,595
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(5)
Less than 12 months, fair value	495
12 months or more, unrealized gross losses	0
12 months or more, fair value	0
Total unrealized gross losses	(5)
Total, fair value	495
Agency pass-through certificates
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(8,768)	(17,683)
Less than 12 months, fair value	583,301	1,163,358
12 months or more, unrealized gross losses	(48,442)	(2,577)
12 months or more, fair value	1,428,150	249,304
Total unrealized gross losses	(57,210)	(20,260)
Total, fair value	$ 2,011,451	$ 1,412,662

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous 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
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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