Investment Securities Gross Unrealized Losses and Fair Values of Investments by Category and Length of Time in a Continuous Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 512,674
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(3,968)
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	189,807
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	(25,086)
Estimated Fair Value, Total	702,481
Unrealized Losses, Total	(29,054)
US Government-sponsored Enterprises Debt Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	0
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	80
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	(1)
Estimated Fair Value, Total	80
Unrealized Losses, Total	(1)
State and Municipal Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	15,203
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(170)
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	0
Estimated Fair Value, Total	15,203
Unrealized Losses, Total	(170)
Corporate Debt Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,992
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(4)
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	44,513
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	(6,079)
Estimated Fair Value, Total	47,505
Unrealized Losses, Total	(6,083)
Collateralized Mortgage Obligations [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	457,415
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(3,225)
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	0
Estimated Fair Value, Total	457,415
Unrealized Losses, Total	(3,225)
Collateralized Mortgage Backed Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	25,033
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(7)
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	0
Estimated Fair Value, Total	25,033
Unrealized Losses, Total	(7)
Auction Rate Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	10,954
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(502)
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	143,596
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	(18,825)
Estimated Fair Value, Total	154,550
Unrealized Losses, Total	(19,327)
Debt Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	511,597
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(3,908)
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	188,189
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	(24,905)
Estimated Fair Value, Total	699,786
Unrealized Losses, Total	(28,813)
Equity Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,077
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	(60)
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,618
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	(181)
Estimated Fair Value, Total	2,695
Unrealized Losses, Total	$ (241)