Securities - Amortized cost and fair value of held-to-maturity and available-for-sale securities by contractual maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-Sale, Amortized Cost
Amortized Cost	$ 2,613,609	$ 2,240,607
Available-for-Sale, Fair Value
Fair Value	2,690,448	2,274,581
Held-to-Maturity, Amortized Cost
Amortized Cost	398,373	283,533
Held-to-Maturity, Fair Value
Fair value	413,820	287,904
U.S. Treasuries
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, 1 to 5 years	123,534
Amortized Cost	123,534	152,990
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, 1 to 5 years	128,534
Fair Value	128,534	154,618
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, 1 to 5 years	0
Amortized Cost	0
Held-to-Maturity, Fair Value
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	277
Available-for-Sale, Amortized Cost, 5 to 10 years	26,263
Available-for-Sale, Amortized Cost, More than 10 years	24,772
Amortized Cost	51,312	2,848
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, 1 to 5 years	281
Available-for-Sale, Fair Value, 5 to 10 years	26,561
Available-for-Sale, Fair Value, More than 10 years	24,903
Fair Value	51,745	3,035
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, 1 to 5 years	0
Held-to-Maturity, Amortized Cost, 5 to 10 years	0
Held-to-Maturity, Amortized Cost, More than 10 years	10,616
Amortized Cost	10,616
Held-to-Maturity, Fair Value
Held-to-Maturity, Fair Value, 1 to 5 years	0
Held-to-Maturity, Fair Value, 5 to 10 years	0
Held-to-Maturity, Fair Value, More than 10 years	10,762
Fair value	10,762
State and political subdivisions
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, Within 1 year	20,029
Available-for-Sale, Amortized Cost, 1 to 5 years	39,638
Available-for-Sale, Amortized Cost, 5 to 10 years	51,119
Available-for-Sale, Amortized Cost, More than 10 years	128,404
Amortized Cost	239,190	214,677
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	20,368
Available-for-Sale, Fair Value, 1 to 5 years	42,296
Available-for-Sale, Fair Value, 5 to 10 years	55,609
Available-for-Sale, Fair Value, More than 10 years	140,903
Fair Value	259,176	226,490
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, Within 1 year	200
Held-to-Maturity, Amortized Cost, 1 to 5 years	13,509
Held-to-Maturity, Amortized Cost, 5 to 10 years	9,557
Held-to-Maturity, Amortized Cost, More than 10 years	139,914
Amortized Cost	163,180	45,479
Held-to-Maturity, Fair Value
Held-to-Maturity, Fair Value, Within 1 year	206
Held-to-Maturity, Fair Value, 1 to 5 years	14,722
Held-to-Maturity, Fair Value, 5 to 10 years	10,942
Held-to-Maturity, Fair Value, More than 10 years	143,482
Fair value	169,352	47,044
Corporate bonds
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, Within 1 year	37,498
Available-for-Sale, Amortized Cost, 1 to 5 years	36,373
Available-for-Sale, Amortized Cost, 5 to 10 years	20,111
Available-for-Sale, Amortized Cost, More than 10 years	3,776
Amortized Cost	97,758	202,131
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	37,759
Available-for-Sale, Fair Value, 1 to 5 years	37,316
Available-for-Sale, Fair Value, 5 to 10 years	20,380
Available-for-Sale, Fair Value, More than 10 years	3,815
Fair Value	99,270	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
Held-to-Maturity, Amortized Cost, More than 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
Held-to-Maturity, Fair Value, More than 10 years	0
Fair value	0
Asset-backed securities
Available-for-Sale, Amortized Cost
Amortized Cost	458,741	104,298
Available-for-Sale, Fair Value
Fair Value	457,402	$ 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	1,256,734
Available-for-Sale, Fair Value
Fair Value	1,299,804
Held-to-Maturity, Amortized Cost
Amortized Cost	125,011
Held-to-Maturity, Fair Value
Fair value	130,160
Commercial mortgage-backed securities
Available-for-Sale, Amortized Cost
Amortized Cost	386,340
Available-for-Sale, Fair Value
Fair Value	394,517
Held-to-Maturity, Amortized Cost
Amortized Cost	99,566
Held-to-Maturity, Fair Value
Fair value	$ 103,546