Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Securities in the available-for-sale portfolio in a continuous loss position
Number of individual securities in a continuous loss position | security	202	196
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	$ (58,638)	$ (29,116)
12 Months or Longer Unrealized Losses	(130,877)	(93,742)
Total Unrealized Losses	(189,515)	(122,858)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	2,114,630	2,119,812
12 Months or Longer Fair Value	2,913,181	3,052,744
Total Fair Value	5,027,811	5,172,556
U.S. Treasury securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1,713)	(994)
12 Months or Longer Unrealized Losses	(14,702)	(11,127)
Total Unrealized Losses	(16,415)	(12,121)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	47,500	48,182
12 Months or Longer Fair Value	340,147	344,073
Total Fair Value	387,647	392,255
Government-sponsored enterprises debt securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1,299)	(642)
12 Months or Longer Unrealized Losses	(8,760)	(6,469)
Total Unrealized Losses	(10,059)	(7,111)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	58,701	59,358
12 Months or Longer Fair Value	180,954	183,243
Total Fair Value	239,655	242,601
Government agency mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(6,345)	(976)
12 Months or Longer Unrealized Losses	(7,889)	(4,492)
Total Unrealized Losses	(14,234)	(5,468)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	272,336	200,963
12 Months or Longer Fair Value	140,778	150,427
Total Fair Value	413,114	351,390
Government-sponsored enterprises mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(1)
12 Months or Longer Unrealized Losses	(6,463)	(4,129)
Total Unrealized Losses	(6,463)	(4,130)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	199	63
12 Months or Longer Fair Value	162,089	168,342
Total Fair Value	162,288	168,405
Collateralized mortgage obligations: Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(41,628)	(23,236)
12 Months or Longer Unrealized Losses	(75,554)	(53,510)
Total Unrealized Losses	(117,182)	(76,746)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	1,377,239	1,473,170
12 Months or Longer Fair Value	1,710,469	1,803,338
Total Fair Value	3,087,708	3,276,508
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(7,092)	(3,203)
12 Months or Longer Unrealized Losses	(17,509)	(14,015)
Total Unrealized Losses	(24,601)	(17,218)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	338,739	327,435
12 Months or Longer Fair Value	378,744	403,321
Total Fair Value	717,483	730,756
States and political subdivisions
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(561)	(64)
Total Unrealized Losses	(561)	(64)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	19,916	10,641
Total Fair Value	$ 19,916	$ 10,641