Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 1,264,060	$ 1,256,587
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	4,424	13,829
Gross unrealized loss	(13,083)	(4,207)
Fair Value, Available-for-sale securities
Available-for-sale securities, at fair value	$ 1,255,401	$ 1,266,209
Yield, Available-for-sale securities
Yield	3.28%	2.86%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, at amortized cost	$ 1,670,450	$ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, Gross unrealized gains	635	7,143
Held-to-maturity securities ,Gross unrealized losses	(61,051)	(18,086)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, Fair Value	$ 1,610,034	$ 1,635,913
Yield, Held-to-maturity securities
Held-to-maturity securities, Yield	3.16%	3.14%
Investments	$ 2,934,510	$ 2,903,443
Gross unrealized gains on investments	5,059	20,972
Gross unrealized losses on investments	(74,134)	(22,293)
Total financial assets	$ 2,865,435	$ 2,902,122
Yield on investments	3.21%	3.02%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Within 1 year		$ 9,300
1 to 5 years		5,688
5 to 10 years	$ 63,986	69,108
Over 10 years	153,928	127,936
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year		146
Gross unrealized gains, 1 to 5 years		2
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized gains, Over 10 years	122	353
Gross unrealized losses, Within 1 year		0
Gross unrealized losses, 1 to 5 years		0
Gross unrealized losses, 5 to 10 years	(1,678)	(1,238)
Gross unrealized losses, Over 10 years	(349)	(218)
Fair Value, Available-for-sale securities
Within 1 year		9,446
1 to 5 years		5,690
5 to 10 years	62,308	67,870
Over 10 years	$ 153,701	$ 128,071
Yield, Available-for-sale securities
Within 1 year		10.38%
1 to 5 years		1.51%
5 to 10 years	2.41%	1.93%
Over 10 years	2.71%	1.92%
Equity securities
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 500	$ 500
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	22
Gross unrealized losses, 1 to 5 years	(9)	0
Fair Value, Available-for-sale securities
1 to 5 years	$ 491	$ 522
Yield, Available-for-sale securities
1 to 5 years	1.80%	1.80%
Corporate debt securities
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 113,727	$ 63,622
5 to 10 years	69,964	119,960
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	1,687	2,083
Gross unrealized gains, 5 to 10 years	0	210
Gross unrealized losses, 1 to 5 years	(273)	0
Gross unrealized losses, 5 to 10 years	(873)	(577)
Fair Value, Available-for-sale securities
1 to 5 years	115,141	65,705
5 to 10 years	$ 69,091	$ 119,593
Yield, Available-for-sale securities
1 to 5 years	3.60%	2.96%
5 to 10 years	3.23%	2.62%
Municipal bonds
Amortized Cost, Available-for-sale securities
Within 1 year		$ 2,344
1 to 5 years	$ 1,390	1,367
Over 10 years	20,328	20,343
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year		10
Gross unrealized gains, 1 to 5 years	0	55
Gross unrealized gains, Over 10 years	1,047	2,505
Gross unrealized losses, Within 1 year		0
Gross unrealized losses, 1 to 5 years	(10)	0
Gross unrealized losses, Over 10 years	0	0
Fair Value, Available-for-sale securities
Within 1 year		2,354
1 to 5 years	1,380	1,422
Over 10 years	$ 21,375	$ 22,848
Yield, Available-for-sale securities
Within 1 year		1.23%
1 to 5 years	2.05%	2.05%
Over 10 years	6.45%	6.45%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Without single maturity date	$ 836,777	$ 828,069
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	1,563	8,402
Gross unrealized losses, Without single maturity date	(9,891)	(2,174)
Fair Value, Available-for-sale securities
Without single maturity date	$ 828,449	$ 834,297
Yield, Available-for-sale securities
Without single maturity date	3.34%	2.96%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, at amortized cost	$ 1,655,450	$ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, Gross unrealized gains	635	7,143
Held-to-maturity securities ,Gross unrealized losses	(61,051)	(18,086)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, Fair Value	$ 1,595,034	$ 1,635,913
Yield, Held-to-maturity securities
Held-to-maturity securities, Yield	3.16%	3.14%
Commercial MBS
Amortized Cost, Available-for-sale securities
Without single maturity date	$ 3,460	$ 8,350
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	5	41
Gross unrealized losses, Without single maturity date	0	0
Fair Value, Available-for-sale securities
Without single maturity date	$ 3,465	$ 8,391
Yield, Available-for-sale securities
Without single maturity date	4.27%	3.31%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, at amortized cost	$ 15,000
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, Gross unrealized gains	0
Held-to-maturity securities ,Gross unrealized losses	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, Fair Value	$ 15,000
Yield, Held-to-maturity securities
Held-to-maturity securities, Yield	2.94%