Investment Securities Gross Unrealized Losses and Fair Values of Investments by Category and Length of Time in a Continuous Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Estimated Fair Value, Less than 12 Months	$ 800,776
Unrealized Losses, Less than 12 Months	(6,400)
Estimated Fair Value, 12 Months or Longer	662,705
Unrealized Losses, 12 Months or Longer	(31,626)
Estimated Fair Value, Total	1,463,481
Unrealized Losses, Total	(38,026)
U.S. Government sponsored agency securities
Estimated Fair Value, Less than 12 Months	9,957
Unrealized Losses, Less than 12 Months	(53)
Estimated Fair Value, 12 Months or Longer	0
Unrealized Losses, 12 Months or Longer	0
Estimated Fair Value, Total	9,957
Unrealized Losses, Total	(53)
Corporate debt securities
Estimated Fair Value, Less than 12 Months	12,892
Unrealized Losses, Less than 12 Months	(97)
Estimated Fair Value, 12 Months or Longer	33,036
Unrealized Losses, 12 Months or Longer	(5,979)
Estimated Fair Value, Total	45,928
Unrealized Losses, Total	(6,076)
Collateralized mortgage obligations
Estimated Fair Value, Less than 12 Months	166,007
Unrealized Losses, Less than 12 Months	(1,467)
Estimated Fair Value, 12 Months or Longer	467,778
Unrealized Losses, 12 Months or Longer	(15,505)
Estimated Fair Value, Total	633,785
Unrealized Losses, Total	(16,972)
Mortgage-backed securities
Estimated Fair Value, Less than 12 Months	611,920
Unrealized Losses, Less than 12 Months	(4,783)
Estimated Fair Value, 12 Months or Longer	63,818
Unrealized Losses, 12 Months or Longer	(1,421)
Estimated Fair Value, Total	675,738
Unrealized Losses, Total	(6,204)
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	98,059
Unrealized Losses, 12 Months or Longer	(8,713)
Estimated Fair Value, Total	98,059
Unrealized Losses, Total	(8,713)
Debt securities
Estimated Fair Value, Less than 12 Months	800,776
Unrealized Losses, Less than 12 Months	(6,400)
Estimated Fair Value, 12 Months or Longer	662,691
Unrealized Losses, 12 Months or Longer	(31,618)
Estimated Fair Value, Total	1,463,467
Unrealized Losses, Total	(38,018)
Equity securities
Estimated Fair Value, Less than 12 Months	0
Unrealized Losses, Less than 12 Months	0
Estimated Fair Value, 12 Months or Longer	14
Unrealized Losses, 12 Months or Longer	(8)
Estimated Fair Value, Total	14
Unrealized Losses, Total	$ (8)