Fair Value Measurements (Investments by Contractual Maturity Date) (Details) (USD $)	3 Months Ended	9 Months Ended
	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Sep. 30, 2013
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 3,064,410,000	$ 3,064,410,000		$ 2,350,864,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	42,097,000	42,097,000		19,686,000
Gross unrealized loss	(3,486,000)	(3,486,000)		(9,602,000)
Fair Value, Available-for-sale securities
Fair Value	3,103,021,000	3,103,021,000		2,360,948,000
Yield, Available-for-sale securities
Yield	2.09%	2.09%		1.70%
Yield, Held-to-maturity securities
Investments	4,648,263,000	4,648,263,000		4,005,530,000
Gross unrealized gains on investments	48,107,000	48,107,000		23,073,000
Gross unrealized losses on investments	(59,109,000)	(59,109,000)		(84,806,000)
Fair value of investments	4,637,261,000	4,637,261,000		3,943,797,000
Yield on investments	2.46%	2.46%		2.30%
Available-for-sale securities sold	0	0	43,198,000
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	111,002,000	111,002,000		61,002,000
5 to 10 years	140,989,000	140,989,000		129,219,000
Over 10 years	465,229,000	465,229,000		344,571,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	2,834,000	2,834,000		3,393,000
Gross unrealized gains, 5 to 10 years	562,000	562,000		0
Gross unrealized gains, Over 10 years	1,761,000	1,761,000		0
Gross unrealized losses, 1 to 5 years	(381,000)	(381,000)		(252,000)
Gross unrealized losses, 5 to 10 years	(62,000)	(62,000)		(1,547,000)
Gross unrealized losses, Over 10 years	(622,000)	(622,000)		(2,411,000)
Fair Value, Available-for-sale securities
1 to 5 years	113,455,000	113,455,000		64,143,000
5 to 10 years	141,489,000	141,489,000		127,672,000
Over 10 years	466,368,000	466,368,000		342,160,000
Fair Value	721,312,000	721,312,000		533,975,000
Yield, Available-for-sale securities
1 to 5 years	1.57%	1.57%		1.98%
5 to 10 years	1.55%	1.55%		0.86%
Over 10 years	1.56%	1.56%		0.93%
Equity Securities [Member]
Amortized Cost, Available-for-sale securities
Within 1 year	500,000	500,000
1 to 5 years	100,000,000	100,000,000		500,000
5 to 10 years	0	0		100,000,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	13,000	13,000
Gross unrealized gains, 1 to 5 years	1,468,000	1,468,000		11,000
Gross unrealized gains, 5 to 10 years	0	0		726,000
Gross unrealized losses, Within 1 year	0	0
Gross unrealized losses, 1 to 5 years	0	0		0
Gross unrealized losses, 5 to 10 years	0	0		0
Fair Value, Available-for-sale securities
Within 1 year	513,000	513,000
1 to 5 years	101,468,000	101,468,000		511,000
5 to 10 years	0	0		100,726,000
Fair Value	101,981,000	101,981,000		101,237,000
Yield, Available-for-sale securities
Within 1 year	1.80%	1.80%
1 to 5 years	1.90%	1.90%		2.17%
5 to 10 years	0.00%	0.00%		1.80%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Amortized Cost, Available-for-sale securities
Without single maturity date	1,636,366,000	1,636,366,000		1,245,400,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	27,896,000	27,896,000		10,270,000
Gross unrealized losses, Without single maturity date	(2,261,000)	(2,261,000)		(4,494,000)
Fair Value, Available-for-sale securities
Without single maturity date	1,662,001,000	1,662,001,000		1,251,176,000
Fair Value	1,662,001,000	1,662,001,000		1,251,176,000
Yield, Available-for-sale securities
Without single maturity date	2.59%	2.59%		2.18%
Amortized Cost, Held-to-maturity securities
Without single maturity date	1,583,853,000	1,583,853,000		1,654,666,000
Gross Unrealized Gains / Losses, Held-to-maturity securities
Gross unrealized gains, Without single maturity date	6,010,000	6,010,000		3,387,000
Gross unrealized losses, Without single maturity date	(55,623,000)	(55,623,000)		(75,204,000)
Fair Value, Held-to-maturity securities
Without single maturity date	1,534,240,000	1,534,240,000		1,582,849,000
Yield, Held-to-maturity securities
Without a single maturity date	3.13%	3.13%		3.14%
Other Commercial Mortgage-Backed Securities [Member]
Amortized Cost, Available-for-sale securities
Without single maturity date	109,300,000	109,300,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	67,000	67,000
Gross unrealized losses, Without single maturity date	0	0
Fair Value, Available-for-sale securities
Without single maturity date	109,367,000	109,367,000
Fair Value	109,367,000	109,367,000		0
Yield, Available-for-sale securities
Without single maturity date	1.69%	1.69%
Corporate Debt Securities [Member]
Amortized Cost, Available-for-sale securities
Within 1 year	0	0		19,500,000
1 to 5 years	317,452,000	317,452,000		317,190,000
5 to 10 years	113,165,000	113,165,000		113,060,000
Over 10 years	50,000,000	50,000,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	0	0		3,000
Gross unrealized gains, 1 to 5 years	2,562,000	2,562,000		1,980,000
Gross unrealized gains, 5 to 10 years	2,064,000	2,064,000		1,180,000
Gross unrealized gains, Over 10 years	0	0
Gross unrealized losses, Within 1 year	0	0		0
Gross unrealized losses, 1 to 5 years	0	0		(130,000)
Gross unrealized losses, 5 to 10 years	(160,000)	(160,000)		(768,000)
Gross unrealized losses, Over 10 years	0	0
Fair Value, Available-for-sale securities
Within 1 year	0	0		19,503,000
1 to 5 years	320,014,000	320,014,000		319,040,000
5 to 10 years	115,069,000	115,069,000		113,472,000
Over 10 years	50,000,000	50,000,000
Fair Value	485,083,000	485,083,000		452,015,000
Yield, Available-for-sale securities
Within 1 year	0.00%	0.00%		0.49%
1 to 5 years	0.72%	0.72%		0.75%
5 to 10 years	1.49%	1.49%		1.53%
Over 10 years	3.00%	3.00%
Municipal Bonds [Member]
Amortized Cost, Available-for-sale securities
Over 10 years	20,407,000	20,407,000		20,422,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Over 10 years	2,870,000	2,870,000		2,123,000
Gross unrealized losses, Over 10 years	0	0		0
Fair Value, Available-for-sale securities
Over 10 years	$ 23,277,000	$ 23,277,000		$ 22,545,000
Yield, Available-for-sale securities
Over 10 years	6.45%	6.45%		6.45%