Fair Value Measurements (Reconciliation Of Amortized Cost of Fair Value of Investments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2012
Available-for-Sale Securities, Amortized Cost
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	$ 1,985,376		$ 1,760,668
Available-for-Sale Securities, Gross Unrealized Gains (Losses)
Available-for-sale Securities, Gross Unrealized Gains	19,382		22,161
Available-for-sale Securities, Gross Unrealized Losses	981		1,124
Available-for-Sale Securities, Fair Value
Available-for-sale Securities, Debt Securities	2,003,777		1,781,705
Available-for-Sale Securities, Yield
Available-for-sale Securities, Yield	4.11%		1.99%
Held-to-maturity Securities, Yield	3.05%		3.10%
Yield	2.26%		2.44%
Held-to-Maturity Securities, Amortized Cost
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount	1,407,246		1,191,487
Held-to-Maturity Securities, Gross Unrealized Gains
Held-to-maturity Securities, Unrecognized Holding Gain	16,616		25,736
Held-to-Maturity Securities, Gross Unrealized Losses
Held-to-maturity Securities, Unrecognized Holding Loss	(291)		0
Held-to-Maturity Securities, Fair Value
Held-to-maturity Securities, Debt Maturities, Fair Value	1,423,571		1,217,223
Investments	3,392,622		2,952,155
Gross Unrealized Gains on Investments	35,998		47,897
Gross Unrealized Losses on Investments	1,272		1,124
Investments, Fair Value Disclosure	3,427,348		2,998,928
Available-for-sale securities sold	43,899	3,500
US Treasury and Government [Member]
Available-for-Sale Securities, Yield
1 to 5 years	0.00%		0.00%
Held-to-Maturity Securities, Amortized Cost
5 to 10 years	0		0
Held-to-Maturity Securities, Gross Unrealized Gains
1 to 5 years	0		0
Held-to-Maturity Securities, Gross Unrealized Losses
1 to 5 years	0		0
Held-to-Maturity Securities, Fair Value
1 to 5 years	0		0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available-for-Sale Securities, Yield
Agency pass-through certificates	3.05%		3.10%
Held-to-Maturity Securities, Amortized Cost
Agency pass-through certificates	1,407,246		1,190,692
Held-to-Maturity Securities, Gross Unrealized Gains
Agency pass-through certificates	16,616		25,729
Held-to-Maturity Securities, Gross Unrealized Losses
Agency pass-through certificates	(291)		0
Held-to-Maturity Securities, Fair Value
Agency pass-through certificates	1,423,571		1,216,421
Municipal Bonds [Member]
Available-for-Sale Securities, Gross Unrealized Gains (Losses)
Within 1 year	0		7
Within 1 year	0		0
Available-for-Sale Securities, Yield
Within 1 year	0.00%		5.80%
1 to 5 years	0.00%		0.00%
5 to 10 years	0.00%		0.00%
Over 10 years	0.00%		0.00%
Held-to-Maturity Securities, Amortized Cost
Within 1 year	0		795
5 to 10 years	0		0
5 to 10 years	0		0
Over 10 years	0		0
Held-to-Maturity Securities, Gross Unrealized Gains
1 to 5 years	0		0
5 to 10 years	0		0
Over 10 years	0		0
Held-to-Maturity Securities, Gross Unrealized Losses
1 to 5 years	0		0
5 to 10 years	0		0
Over 10 years	0		0
Held-to-Maturity Securities, Fair Value
Within 1 year	0		802
1 to 5 years	0		0
5 to 10 years	0		0
Over 10 years	0		0
Municipal Bonds [Member]
Available-for-Sale Securities, Amortized Cost
Over 10 years	20,437		20,442
Available-for-Sale Securities, Gross Unrealized Gains (Losses)
Over 10 years	4,623		4,402
Over 10 years	0		0
Available-for-Sale Securities, Fair Value
Over 10 years	25,060		24,844
Available-for-Sale Securities, Yield
Over 10 years	6.45%		6.45%
US Treasury and Government [Member]
Available-for-Sale Securities, Amortized Cost
Within 1 year	18,133		19,999
1 to 5 years	58,000		0
5 to 10 years	33,300		59,300
Over 10 years	293,922		100,000
Available-for-Sale Securities, Gross Unrealized Gains (Losses)
Within 1 year	36		42
1 to 5 years	2,597		0
5 to 10 years	1,435		4,225
Over 10 years	0		0
Within 1 year	(51)		(6)
1 to 5 years	0		0
5 to 10 years	0		0
Over 10 years	0		0
Available-for-Sale Securities, Fair Value
Within 1 year	18,118		20,035
1 to 5 years	60,597		0
5 to 10 years	34,735		63,525
Over 10 years	293,922		100,000
Available-for-Sale Securities, Yield
Within 1 year	0.44%		0.57%
1 to 5 years	1.55%		0.00%
5 to 10 years	1.75%		2.21%
Over 10 years	0.93%		1.05%
Corporate Debt Securities [Member]
Available-for-Sale Securities, Amortized Cost
Within 1 year	19,500
1 to 5 years	316,928		336,340
5 to 10 years	62,954		62,919
Available-for-Sale Securities, Gross Unrealized Gains (Losses)
Within 1 year	5
1 to 5 years	2,656		2,810
5 to 10 years	1,612		1,324
Within 1 year	0
1 to 5 years	(292)		(61)
5 to 10 years	(6)		(7)
Available-for-Sale Securities, Fair Value
Within 1 year	19,505
1 to 5 years	319,292		339,089
5 to 10 years	64,560		64,236
Available-for-Sale Securities, Yield
Within 1 year	0.49%
1 to 5 years	0.84%		0.91%
5 to 10 years	2.02%		2.73%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available-for-Sale Securities, Amortized Cost
Agency pass-through certificates	1,162,202		1,161,668
Available-for-Sale Securities, Gross Unrealized Gains (Losses)
Agency pass-through certificates	6,418		9,358
Agency pass-through certificates	(632)		(1,050)
Available-for-Sale Securities, Fair Value
Agency pass-through certificates	$ 1,167,988		$ 1,169,976
Available-for-Sale Securities, Yield
Agency pass-through certificates	2.07%		2.28%