Securities Available for Sale and Held to Maturity - Securities with Gross Unrealized Losses, Segregated by Length of Impairment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 23,579	$ 98,188
Estimated Fair Value, 12 Months or More	2,123,233	2,399,747
Estimated Fair Value, Total	2,146,812	2,497,935
Gross Unrealized (Losses), Less than 12 Months	(107)	(382)
Gross Unrealized (Losses), 12 Months or More	(37,120)	(63,994)
Gross Unrealized (Losses), Total	(37,227)	(64,376)
U.S. Government Agency Obligations [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	0	0
Estimated Fair Value, 12 Months or More	23,223	25,045
Estimated Fair Value, Total	23,223	25,045
Gross Unrealized (Losses), Less than 12 Months	0	0
Gross Unrealized (Losses), 12 Months or More	(993)	(1,009)
Gross Unrealized (Losses), Total	(993)	(1,009)
Obligations of States and Political Subdivisions [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,366	4,954
Estimated Fair Value, 12 Months or More	11,095	12,802
Estimated Fair Value, Total	12,461	17,756
Gross Unrealized (Losses), Less than 12 Months	(6)	(9)
Gross Unrealized (Losses), 12 Months or More	(144)	(157)
Gross Unrealized (Losses), Total	(150)	(166)
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	3,117	9,163
Estimated Fair Value, 12 Months or More	57,808	61,141
Estimated Fair Value, Total	60,925	70,304
Gross Unrealized (Losses), Less than 12 Months	(12)	(54)
Gross Unrealized (Losses), 12 Months or More	(1,471)	(2,504)
Gross Unrealized (Losses), Total	(1,483)	(2,558)
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	16	31,931
Estimated Fair Value, 12 Months or More	703,426	731,749
Estimated Fair Value, Total	703,442	763,680
Gross Unrealized (Losses), Less than 12 Months	(1)	(172)
Gross Unrealized (Losses), 12 Months or More	(13,219)	(21,678)
Gross Unrealized (Losses), Total	(13,220)	(21,850)
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	4,548	46,643
Estimated Fair Value, 12 Months or More	1,098,914	1,296,221
Estimated Fair Value, Total	1,103,462	1,342,864
Gross Unrealized (Losses), Less than 12 Months	(5)	(110)
Gross Unrealized (Losses), 12 Months or More	(19,060)	(33,926)
Gross Unrealized (Losses), Total	(19,065)	(34,036)
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	14,532	5,497
Estimated Fair Value, 12 Months or More	228,767	272,789
Estimated Fair Value, Total	243,299	278,286
Gross Unrealized (Losses), Less than 12 Months	(83)	(37)
Gross Unrealized (Losses), 12 Months or More	(2,233)	(4,720)
Gross Unrealized (Losses), Total	$ (2,316)	$ (4,757)