Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 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	203	196
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	$ (62,384)	$ (29,116)
12 Months or Longer Unrealized Losses	(151,730)	(93,742)
Total Unrealized Losses	(214,114)	(122,858)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	1,865,793	2,119,812
12 Months or Longer Fair Value	2,971,414	3,052,744
Total Fair Value	4,837,207	5,172,556
U.S. Treasury securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1,963)	(994)
12 Months or Longer Unrealized Losses	(15,724)	(11,127)
Total Unrealized Losses	(17,687)	(12,121)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	47,287	48,182
12 Months or Longer Fair Value	338,770	344,073
Total Fair Value	386,057	392,255
Government-sponsored enterprises debt securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1,700)	(642)
12 Months or Longer Unrealized Losses	(9,487)	(6,469)
Total Unrealized Losses	(11,187)	(7,111)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	58,300	59,358
12 Months or Longer Fair Value	180,227	183,243
Total Fair Value	238,527	242,601
Government agency mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(8,499)	(976)
12 Months or Longer Unrealized Losses	(8,420)	(4,492)
Total Unrealized Losses	(16,919)	(5,468)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	304,708	200,963
12 Months or Longer Fair Value	134,142	150,427
Total Fair Value	438,850	351,390
Government-sponsored enterprises mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1)	(1)
12 Months or Longer Unrealized Losses	(7,018)	(4,129)
Total Unrealized Losses	(7,019)	(4,130)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	189	63
12 Months or Longer Fair Value	155,708	168,342
Total Fair Value	155,897	168,405
Collateralized mortgage obligations: Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(42,264)	(23,236)
12 Months or Longer Unrealized Losses	(90,796)	(53,510)
Total Unrealized Losses	(133,060)	(76,746)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	1,162,582	1,473,170
12 Months or Longer Fair Value	1,762,224	1,803,338
Total Fair Value	2,924,806	3,276,508
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(7,362)	(3,203)
12 Months or Longer Unrealized Losses	(20,285)	(14,015)
Total Unrealized Losses	(27,647)	(17,218)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	272,911	327,435
12 Months or Longer Fair Value	400,343	403,321
Total Fair Value	673,254	730,756
States and political subdivisions
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(595)	(64)
Total Unrealized Losses	(595)	(64)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	19,816	10,641
Total Fair Value	$ 19,816	$ 10,641