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	Dec. 31, 2014
Estimated Fair Value, Less than 12 Months	$ 91,151
Unrealized Losses, Less than 12 Months	(241)
Estimated Fair Value, 12 Months or Longer	984,994
Unrealized Losses, 12 Months or Longer	(37,687)
Estimated Fair Value, Total	1,076,145
Unrealized Losses, Total	(37,928)
State and municipal securities
Estimated Fair Value, Less than 12 Months	3,282
Unrealized Losses, Less than 12 Months	(4)
Estimated Fair Value, 12 Months or Longer	19,640
Unrealized Losses, 12 Months or Longer	(262)
Estimated Fair Value, Total	22,922
Unrealized Losses, Total	(266)
Corporate debt securities
Estimated Fair Value, Less than 12 Months	4,952
Unrealized Losses, Less than 12 Months	(17)
Estimated Fair Value, 12 Months or Longer	36,849
Unrealized Losses, 12 Months or Longer	(6,091)
Estimated Fair Value, Total	41,801
Unrealized Losses, Total	(6,108)
Collateralized mortgage obligations
Estimated Fair Value, Less than 12 Months	46,121
Unrealized Losses, Less than 12 Months	(179)
Estimated Fair Value, 12 Months or Longer	592,119
Unrealized Losses, 12 Months or Longer	(20,608)
Estimated Fair Value, Total	638,240
Unrealized Losses, Total	(20,787)
Mortgage-backed securities
Estimated Fair Value, Less than 12 Months	36,791
Unrealized Losses, Less than 12 Months	(40)
Estimated Fair Value, 12 Months or Longer	235,368
Unrealized Losses, 12 Months or Longer	(2,904)
Estimated Fair Value, Total	272,159
Unrealized Losses, Total	(2,944)
Auction rate securities
Estimated Fair Value, Less than 12 Months	0
Unrealized Losses, Less than 12 Months	0
Estimated Fair Value, 12 Months or Longer	100,941
Unrealized Losses, 12 Months or Longer	(7,810)
Estimated Fair Value, Total	100,941
Unrealized Losses, Total	(7,810)
Debt securities
Estimated Fair Value, Less than 12 Months	91,146
Unrealized Losses, Less than 12 Months	(240)
Estimated Fair Value, 12 Months or Longer	984,917
Unrealized Losses, 12 Months or Longer	(37,675)
Estimated Fair Value, Total	1,076,063
Unrealized Losses, Total	(37,915)
Equity securities
Estimated Fair Value, Less than 12 Months	5
Unrealized Losses, Less than 12 Months	(1)
Estimated Fair Value, 12 Months or Longer	77
Unrealized Losses, 12 Months or Longer	(12)
Estimated Fair Value, Total	82
Unrealized Losses, Total	$ (13)

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