Securities - Amortized cost and fair value of held-to-maturity and available-for-sale securities by contractual maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale, Amortized Cost
Amortized Cost	$ 2,202,217	$ 2,240,607
Available-for-Sale, Fair Value
Fair Value	2,249,876	2,274,581
Held-to-Maturity, Amortized Cost
Amortized Cost	290,404	283,533
Held-to-Maturity, Fair Value
Fair value	301,595	287,904
U.S. Treasuries
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, Within 1 year	29,967
Available-for-Sale, Amortized Cost, 1 to 5 years	123,252
Amortized Cost	153,219	152,990
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	30,035
Available-for-Sale, Fair Value, 1 to 5 years	128,929
Fair Value	158,964	154,618
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, Within 1 year	0
Held-to-Maturity, Amortized Cost, 1 to 5 years	0
Amortized Cost	0
Held-to-Maturity, Fair Value
Held-to-Maturity, Fair Value, Within 1 year	0
Held-to-Maturity, Fair Value, 1 to 5 years	0
Fair value	0
U.S. Government agencies
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, 1 to 5 years	354
Available-for-Sale, Amortized Cost, More than 10 years	2,423
Amortized Cost	2,777	2,848
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, 1 to 5 years	358
Available-for-Sale, Fair Value, More than 10 years	2,609
Fair Value	2,967	3,035
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, 1 to 5 years	0
Held-to-Maturity, Amortized Cost, More than 10 years	0
Amortized Cost	0
Held-to-Maturity, Fair Value
Held-to-Maturity, Fair Value, 1 to 5 years	0
Held-to-Maturity, Fair Value, More than 10 years	0
Fair value	0
State and political subdivisions
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, Within 1 year	935
Available-for-Sale, Amortized Cost, 1 to 5 years	54,097
Available-for-Sale, Amortized Cost, 5 to 10 years	24,255
Available-for-Sale, Amortized Cost, More than 10 years	134,904
Amortized Cost	214,191	214,677
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	935
Available-for-Sale, Fair Value, 1 to 5 years	55,202
Available-for-Sale, Fair Value, 5 to 10 years	25,802
Available-for-Sale, Fair Value, More than 10 years	145,742
Fair Value	227,681	226,490
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, Within 1 year	1,350
Held-to-Maturity, Amortized Cost, 1 to 5 years	11,759
Held-to-Maturity, Amortized Cost, 5 to 10 years	13,323
Held-to-Maturity, Amortized Cost, More than 10 years	34,651
Amortized Cost	61,083	45,479
Held-to-Maturity, Fair Value
Held-to-Maturity, Fair Value, Within 1 year	1,363
Held-to-Maturity, Fair Value, 1 to 5 years	12,421
Held-to-Maturity, Fair Value, 5 to 10 years	14,714
Held-to-Maturity, Fair Value, More than 10 years	35,962
Fair value	64,460	47,044
Corporate bonds
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, Within 1 year	160,000
Available-for-Sale, Amortized Cost, 1 to 5 years	27,570
Available-for-Sale, Amortized Cost, 5 to 10 years	3,500
Amortized Cost	191,070	202,131
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	159,258
Available-for-Sale, Fair Value, 1 to 5 years	27,835
Available-for-Sale, Fair Value, 5 to 10 years	3,692
Fair Value	190,785	203,091
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, Within 1 year	0
Held-to-Maturity, Amortized Cost, 1 to 5 years	0
Held-to-Maturity, Amortized Cost, 5 to 10 years	0
Amortized Cost	0
Held-to-Maturity, Fair Value
Held-to-Maturity, Fair Value, Within 1 year	0
Held-to-Maturity, Fair Value, 1 to 5 years	0
Held-to-Maturity, Fair Value, 5 to 10 years	0
Fair value	0
Total securities other than mortgage-backed securities
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, Within 1 year	190,902
Available-for-Sale, Amortized Cost, 1 to 5 years	205,273
Available-for-Sale, Amortized Cost, 5 to 10 years	27,755
Available-for-Sale, Amortized Cost, More than 10 years	137,327
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	190,228
Available-for-Sale, Fair Value, 1 to 5 years	212,324
Available-for-Sale, Fair Value, 5 to 10 years	29,494
Available-for-Sale, Fair Value, More than 10 years	148,351
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, Within 1 year	1,350
Held-to-Maturity, Amortized Cost, 1 to 5 years	11,759
Held-to-Maturity, Amortized Cost, 5 to 10 years	13,323
Held-to-Maturity, Amortized Cost, More than 10 years	34,651
Held-to-Maturity, Fair Value
Held-to-Maturity, Fair Value, Within 1 year	1,363
Held-to-Maturity, Fair Value, 1 to 5 years	12,421
Held-to-Maturity, Fair Value, 5 to 10 years	14,714
Held-to-Maturity, Fair Value, More than 10 years	35,962
Asset-backed securities
Available-for-Sale, Amortized Cost
Amortized Cost	109,542	104,298
Available-for-Sale, Fair Value
Fair Value	104,099	$ 103,369
Held-to-Maturity, Amortized Cost
Amortized Cost	0
Held-to-Maturity, Fair Value
Fair value	0
Residential mortgage-backed securities
Available-for-Sale, Amortized Cost
Amortized Cost	1,269,582
Available-for-Sale, Fair Value
Fair Value	1,297,591
Held-to-Maturity, Amortized Cost
Amortized Cost	145,793
Held-to-Maturity, Fair Value
Fair value	150,490
Commercial mortgage-backed securities
Available-for-Sale, Amortized Cost
Amortized Cost	261,836
Available-for-Sale, Fair Value
Fair Value	267,789
Held-to-Maturity, Amortized Cost
Amortized Cost	83,528
Held-to-Maturity, Fair Value
Fair value	$ 86,645