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	Dec. 31, 2016	Dec. 31, 2015
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)
US Government Agencies Debt Securities [Member]
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)
State and municipal securities
Estimated Fair Value, Less than 12 Months	$ 247,509
Unrealized Losses, Less than 12 Months	(15,676)
Estimated Fair Value, 12 Months or Longer	0
Unrealized Losses, 12 Months or Longer	0
Estimated Fair Value, Total	247,509
Unrealized Losses, Total	(15,676)
Corporate debt securities
Estimated Fair Value, Less than 12 Months	11,922	12,892
Unrealized Losses, Less than 12 Months	(110)	(97)
Estimated Fair Value, 12 Months or Longer	34,629	33,036
Unrealized Losses, 12 Months or Longer	(4,475)	(5,979)
Estimated Fair Value, Total	46,551	45,928
Unrealized Losses, Total	(4,585)	(6,076)
Collateralized mortgage obligations
Estimated Fair Value, Less than 12 Months	166,905	166,007
Unrealized Losses, Less than 12 Months	(3,899)	(1,467)
Estimated Fair Value, 12 Months or Longer	258,237	467,778
Unrealized Losses, 12 Months or Longer	(8,279)	(15,505)
Estimated Fair Value, Total	425,142	633,785
Unrealized Losses, Total	(12,178)	(16,972)
Mortgage-backed securities
Estimated Fair Value, Less than 12 Months	1,137,510	611,920
Unrealized Losses, Less than 12 Months	(17,437)	(4,783)
Estimated Fair Value, 12 Months or Longer	0	63,818
Unrealized Losses, 12 Months or Longer	0	(1,421)
Estimated Fair Value, Total	1,137,510	675,738
Unrealized Losses, Total	(17,437)	(6,204)
Auction rate securities
Estimated Fair Value, Less than 12 Months	0	0
Unrealized Losses, Less than 12 Months	0	0
Estimated Fair Value, 12 Months or Longer	97,256	98,059
Unrealized Losses, 12 Months or Longer	(9,959)	(8,713)
Estimated Fair Value, Total	97,256	98,059
Unrealized Losses, Total	(9,959)	(8,713)
Debt securities
Estimated Fair Value, Less than 12 Months	1,563,846	800,776
Unrealized Losses, Less than 12 Months	(37,122)	(6,400)
Estimated Fair Value, 12 Months or Longer	390,122	662,691
Unrealized Losses, 12 Months or Longer	(22,713)	(31,618)
Estimated Fair Value, Total	1,953,968	1,463,467
Unrealized Losses, Total	$ (59,835)	(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)