Investments (Securities Available for Sale) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,656,308	$ 2,568,512
Gross Unrealized Gains	250,282	262,150
Gross Unrealized Losses	(2,388)	(4,661)
Fair Value	2,904,202	2,826,001
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	118,490	85,196
Less than 12 Months, Unrealized Losses	(1,490)	(1,076)
12 Months or Greater, Fair Value	21,547	21,070
12 Months or Greater, Unrealized Losses	(898)	(3,585)
Total, Fair Value	140,037	106,266
Total, Unrealized Losses	(2,388)	(4,661)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	2,645,982	2,559,052
Gross Unrealized Gains	246,628	259,285
Gross Unrealized Losses	(2,350)	(4,603)
Fair Value	2,890,260	2,813,734	2,890,260	2,813,734
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	118,276	84,440
Less than 12 Months, Unrealized Losses	(1,469)	(1,068)
12 Months or Greater, Fair Value	21,388	20,775
12 Months or Greater, Unrealized Losses	(881)	(3,535)
Total, Fair Value	139,664	105,215
Total, Unrealized Losses	(2,350)	(4,603)
U.S. Agencies [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Unrealized Losses	0	0
U.S. Treasury [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Unrealized Losses	0	0
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	598	599
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(32)	(28)
Fair Value	566	571
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	566	571
Less than 12 Months, Unrealized Losses	(32)	(28)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	566	571
Total, Unrealized Losses	(32)	(28)
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	9,925	15,134
Gross Unrealized Gains	570	932
Gross Unrealized Losses	0	0
Fair Value	10,495	16,066
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Unrealized Losses	0	0
Public Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	257,807	254,853
Gross Unrealized Gains	25,107	26,621
Gross Unrealized Losses	(9)	(47)
Fair Value	282,905	281,427
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	1,008	10,949
Less than 12 Months, Unrealized Losses	(9)	(47)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	1,008	10,949
Total, Unrealized Losses	(9)	(47)
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	2,259,238	2,157,706
Gross Unrealized Gains	211,922	222,587
Gross Unrealized Losses	(1,915)	(2,981)
Fair Value	2,469,245	2,377,312
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	116,702	64,383
Less than 12 Months, Unrealized Losses	(1,428)	(713)
12 Months or Greater, Fair Value	14,475	14,713
12 Months or Greater, Unrealized Losses	(487)	(2,268)
Total, Fair Value	131,177	79,096
Total, Unrealized Losses	(1,915)	(2,981)
Mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	101,497	113,488
Gross Unrealized Gains	8,789	8,905
Gross Unrealized Losses	0	(64)
Fair Value	110,286	122,329
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	3,839
Less than 12 Months, Unrealized Losses	0	(64)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	3,839
Total, Unrealized Losses	0	(64)
Home Equity [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	12,222	12,242
Gross Unrealized Gains	17	0
Gross Unrealized Losses	(394)	(1,483)
Fair Value	11,845	10,759
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	4,698
Less than 12 Months, Unrealized Losses	0	(216)
12 Months or Greater, Fair Value	6,913	6,062
12 Months or Greater, Unrealized Losses	(394)	(1,267)
Total, Fair Value	6,913	10,760
Total, Unrealized Losses	(394)	(1,483)
Manufactured Housing [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Debt securities	4,695	5,030
Gross Unrealized Gains	223	240
Gross Unrealized Losses	0	0
Fair Value	4,918	5,270
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Unrealized Losses	0	0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value			13,942	12,267
Equity Public [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Equity securities	10,326	9,460
Gross Unrealized Gains	3,654	2,865
Gross Unrealized Losses	(38)	(58)
Fair Value	13,942	12,267
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	214	756
Less than 12 Months, Unrealized Losses	(21)	(8)
12 Months or Greater, Fair Value	159	295
12 Months or Greater, Unrealized Losses	(17)	(50)
Total, Fair Value	373	1,051
Total, Unrealized Losses	$ (38)	$ (58)