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, 2014
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,091,977
Available-for-sale Securities, Continuous Unrealized Loss Position Less Than 12 Months, Aggregate Losses Accumulated In Investments	27,371
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	504,345
Unrealized Losses, Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses,	(36,913)
Estimated Fair Value, Total	1,596,322
Available-for-sale Securities, Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	64,284
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
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	50
Estimated Fair Value, Total	50
Municipal Bonds [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	50,286
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	3,996
Estimated Fair Value, Total	54,282
Corporate Debt Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,939
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	38,804
Estimated Fair Value, Total	42,743
Collateralized Mortgage Obligations [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	416,584
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	313,758
Estimated Fair Value, Total	730,342
Collateralized Mortgage Backed Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	621,165
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Estimated Fair Value, Total	621,165
Auction Rate Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	147,619
Estimated Fair Value, Total	147,619
Debt Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,091,974
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	504,227
Estimated Fair Value, Total	1,596,201
Equity Securities [Member]
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3
Estimated Fair Value, Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	118
Estimated Fair Value, Total	121
US Government Agencies Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position Less Than 12 Months, Aggregate Losses Accumulated In Investments	0
Available-for-sale Securities, Continuous Unrealized Loss Position 12 Months Or Longer, Aggregate Losses Accumulated In Investments	1
Available-for-sale Securities, Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	1
Equity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position Less Than 12 Months, Aggregate Losses Accumulated In Investments	1
Available-for-sale Securities, Continuous Unrealized Loss Position 12 Months Or Longer, Aggregate Losses Accumulated In Investments	20
Available-for-sale Securities, Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	21
Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position Less Than 12 Months, Aggregate Losses Accumulated In Investments	27,370
Available-for-sale Securities, Continuous Unrealized Loss Position 12 Months Or Longer, Aggregate Losses Accumulated In Investments	36,893
Available-for-sale Securities, Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	64,263
Auction Rate Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position Less Than 12 Months, Aggregate Losses Accumulated In Investments	0
Available-for-sale Securities, Continuous Unrealized Loss Position 12 Months Or Longer, Aggregate Losses Accumulated In Investments	11,668
Available-for-sale Securities, Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	11,668
Collateralized Mortgage Backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position Less Than 12 Months, Aggregate Losses Accumulated In Investments	11,881
Available-for-sale Securities, Continuous Unrealized Loss Position 12 Months Or Longer, Aggregate Losses Accumulated In Investments	0
Available-for-sale Securities, Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	11,881
Collateralized Mortgage Obligations [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position Less Than 12 Months, Aggregate Losses Accumulated In Investments	13,921
Available-for-sale Securities, Continuous Unrealized Loss Position 12 Months Or Longer, Aggregate Losses Accumulated In Investments	18,948
Available-for-sale Securities, Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	32,869
Corporate Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position Less Than 12 Months, Aggregate Losses Accumulated In Investments	58
Available-for-sale Securities, Continuous Unrealized Loss Position 12 Months Or Longer, Aggregate Losses Accumulated In Investments	6,030
Available-for-sale Securities, Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	6,088
Municipal Bonds [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position Less Than 12 Months, Aggregate Losses Accumulated In Investments	1,510
Available-for-sale Securities, Continuous Unrealized Loss Position 12 Months Or Longer, Aggregate Losses Accumulated In Investments	246
Available-for-sale Securities, Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	$ 1,756