Fair Value Measurements (Investments by Contractual Maturity Date) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2013
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 2,838,197		$ 2,350,864
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	14,428		19,686
Gross unrealized loss	(14,121)		(9,602)
Fair Value, Available-for-sale securities
Fair Value	2,838,504		2,360,948
Yield, Available-for-sale securities
Yield	2.00%		1.70%
Yield, Held-to-maturity securities
Investments	4,469,133		4,005,530
Gross unrealized gains on investments	15,837		23,073
Gross unrealized losses on investments	125,076		84,806
Fair value of investments	4,359,894		3,943,797
Yield on investments	2.41%		2.30%
Available-for-sale securities sold	0	43,899
Term of contractual due dates of substantially all mortgage-backed securities	10 years
US Treasury and Government [Member]
Amortized Cost, Available-for-sale securities
1 to 5 years	61,002		61,002
5 to 10 years	237,101		129,219
Over 10 years	335,626		344,571
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	3,021		3,393
Gross unrealized gains, 5 to 10 years	0		0
Gross unrealized gains, Over 10 years	465		0
Gross unrealized losses, 1 to 5 years	(196)		(252)
Gross unrealized losses, 5 to 10 years	(2,396)		(1,547)
Gross unrealized losses, Over 10 years	(3,297)		(2,411)
Fair Value, Available-for-sale securities
1 to 5 years	63,827		64,143
5 to 10 years	234,705		127,672
Over 10 years	332,794		342,160
Yield, Available-for-sale securities
1 to 5 years	1.98%		1.98%
5 to 10 years	1.64%		0.86%
Over 10 years	0.93%		0.93%
Equity Securities [Member]
Amortized Cost, Available-for-sale securities
Within 1 year	500
1 to 5 years	100,000		500
5 to 10 years	0		100,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	3
Gross unrealized gains, 1 to 5 years	246		11
Gross unrealized gains, 5 to 10 years	0		726
Gross unrealized losses, Within 1 year	0
Gross unrealized losses, 1 to 5 years	0		0
Gross unrealized losses, 5 to 10 years	0		0
Fair Value, Available-for-sale securities
Within 1 year	503
1 to 5 years	100,246		511
5 to 10 years	0		100,726
Yield, Available-for-sale securities
Within 1 year	2.17%
1 to 5 years	1.80%		2.17%
5 to 10 years	0.00%		1.80%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Amortized Cost, Available-for-sale securities
Without single maturity date	1,636,178		1,245,400
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	4,896		10,270
Gross unrealized losses, Without single maturity date	(7,398)		(4,494)
Fair Value, Available-for-sale securities
Without single maturity date	1,633,676		1,251,176
Yield, Available-for-sale securities
Without single maturity date	2.51%		2.18%
Amortized Cost, Held-to-maturity securities
Without single maturity date	1,630,936		1,654,666
Gross Unrealized Gains / Losses, Held-to-maturity securities
Gross unrealized gains, Without single maturity date	1,409		3,387
Gross unrealized losses, Without single maturity date	(110,955)		(75,204)
Fair Value, Held-to-maturity securities
Without single maturity date	1,521,390		1,582,849
Yield, Held-to-maturity securities
Without a single maturity date	3.13%		3.14%
Corporate Debt Securities [Member]
Amortized Cost, Available-for-sale securities
Within 1 year	0		19,500
1 to 5 years	334,278		317,190
5 to 10 years	113,095		113,060
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	0		3
Gross unrealized gains, 1 to 5 years	2,051		1,980
Gross unrealized gains, 5 to 10 years	1,523		1,180
Gross unrealized losses, Within 1 year	0		0
Gross unrealized losses, 1 to 5 years	(93)		(130)
Gross unrealized losses, 5 to 10 years	(741)		(768)
Fair Value, Available-for-sale securities
Within 1 year	0		19,503
1 to 5 years	336,236		319,040
5 to 10 years	113,877		113,472
Yield, Available-for-sale securities
Within 1 year	0.00%		0.49%
1 to 5 years	0.80%		0.75%
5 to 10 years	1.53%		1.53%
Municipal Bonds [Member]
Amortized Cost, Available-for-sale securities
Over 10 years	20,417		20,422
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Over 10 years	2,223		2,123
Gross unrealized losses, Over 10 years	0		0
Fair Value, Available-for-sale securities
Over 10 years	$ 22,640		$ 22,545
Yield, Available-for-sale securities
Over 10 years	6.45%		6.45%