Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 1,540,028
Amortized Cost		$ 1,326,471
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	13,268
Gross unrealized gains		4,685
Gross unrealized loss	(7,690)
Gross unrealized loss		(16,199)
Fair Value, Available-for-sale securities
Available-for-sale securities, at fair value	$ 1,545,606
Available-for-sale securities, at fair value		$ 1,314,957
Yield, Available-for-sale securities
Yield	3.44%
Yield		3.30%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, at amortized cost	$ 1,553,683	$ 1,625,420
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, Gross unrealized gains	3,599	333
Held-to-maturity securities ,Gross unrealized losses	(21,730)	(76,983)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, Fair Value	$ 1,535,552	$ 1,548,770
Yield, Held-to-maturity securities
Held-to-maturity securities, Yield	3.16%	3.16%
Investments	$ 3,093,711	$ 2,951,891
Gross unrealized gains on investments	16,867	5,018
Gross unrealized losses on investments	(29,420)	(93,182)
Total financial assets	$ 3,081,158	$ 2,863,727
Yield on investments	3.30%	3.22%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
5 to 10 years	$ 70,806	$ 60,872
Over 10 years	166,674	148,099
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 5 to 10 years	2	0
Gross unrealized gains, Over 10 years	38	109
Gross unrealized losses, 5 to 10 years	(1,182)	(1,473)
Gross unrealized losses, Over 10 years	(1,058)	(314)
Fair Value, Available-for-sale securities
5 to 10 years	69,626	59,399
Over 10 years	$ 165,654	$ 147,894
Yield, Available-for-sale securities
5 to 10 years	2.75%	2.55%
Over 10 years	3.55%	3.05%
Equity securities
Amortized Cost, Available-for-sale securities
1 to 5 years		$ 500
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years		0
Gross unrealized losses, 1 to 5 years		(12)
Fair Value, Available-for-sale securities
1 to 5 years		$ 488
Yield, Available-for-sale securities
1 to 5 years		1.80%
Corporate debt securities
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 113,832	$ 113,762
5 to 10 years	142,776	69,965
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	1,402	1,875
Gross unrealized gains, 5 to 10 years	808	35
Gross unrealized losses, 1 to 5 years	(263)	(13)
Gross unrealized losses, 5 to 10 years	(574)	(929)
Fair Value, Available-for-sale securities
1 to 5 years	114,971	115,624
5 to 10 years	$ 143,010	$ 69,071
Yield, Available-for-sale securities
1 to 5 years	3.92%	3.59%
5 to 10 years	3.49%	3.23%
Municipal bonds due
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 1,414	$ 1,398
Over 10 years	20,313	20,323
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	12	0
Gross unrealized gains, Over 10 years	669	1,281
Gross unrealized losses, 1 to 5 years	0	(24)
Gross unrealized losses, Over 10 years	0	0
Fair Value, Available-for-sale securities
1 to 5 years	1,426	1,374
Over 10 years	$ 20,982	$ 21,604
Yield, Available-for-sale securities
1 to 5 years	1.92%	2.05%
Over 10 years	6.45%	6.45%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Without single maturity date	$ 1,024,213	$ 908,092
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	10,337	1,383
Gross unrealized losses, Without single maturity date	(4,613)	(13,434)
Fair Value, Available-for-sale securities
Without single maturity date	$ 1,029,937	$ 896,041
Yield, Available-for-sale securities
Without single maturity date	3.36%	3.29%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, at amortized cost	$ 1,538,683	$ 1,610,420
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, Gross unrealized gains	3,599	305
Held-to-maturity securities ,Gross unrealized losses	(21,674)	(76,983)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, Fair Value	$ 1,520,608	$ 1,533,742
Yield, Held-to-maturity securities
Held-to-maturity securities, Yield	3.16%	3.16%
Commercial MBS
Amortized Cost, Available-for-sale securities
Without single maturity date		$ 3,460
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date		2
Gross unrealized losses, Without single maturity date		0
Fair Value, Available-for-sale securities
Without single maturity date		$ 3,462
Yield, Available-for-sale securities
Without single maturity date		4.36%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, at amortized cost	$ 15,000	$ 15,000
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, Gross unrealized gains	0	28
Held-to-maturity securities ,Gross unrealized losses	(56)	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, Fair Value	$ 14,944	$ 15,028
Yield, Held-to-maturity securities
Held-to-maturity securities, Yield	3.35%	3.03%

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).