Investment Securities - Amortized cost and fair value of held-to-maturity and available-for-sale securities by contractual maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-Sale, Amortized Cost
Amortized Cost	$ 2,240,607	$ 2,660,263
Available-for-Sale, Fair Value
Fair Value	2,274,581	2,628,467
Held-to-Maturity, Amortized Cost
Amortized Cost	283,533	274,407
Held-to-Maturity Fair Value
Fair value	287,904	268,803
U.S. Treasuries
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, Within 1 year	29,877
Available-for-Sale, Amortized Cost, 1 to 5 years	123,113
Amortized Cost	152,990	150,712
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	29,962
Available-for-Sale, Fair Value, 1 to 5 years	124,656
Fair Value	154,618	149,307
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
US Government agencies
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, 1 to 5 years	405
Available-for-Sale, Amortized Cost, More than 10 years	2,443
Amortized Cost	2,848
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, 1 to 5 years	404
Available-for-Sale, Fair Value, More than 10 years	2,631
Fair Value	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	54,102
Available-for-Sale, Amortized Cost, 5 to 10 years	22,585
Available-for-Sale, Amortized Cost, More than 10 years	137,055
Amortized Cost	214,677	234,750
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	939
Available-for-Sale, Fair Value, 1 to 5 years	55,491
Available-for-Sale, Fair Value, 5 to 10 years	23,766
Available-for-Sale, Fair Value, More than 10 years	146,294
Fair Value	226,490	233,941
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, Within 1 year	1,350
Held-to-Maturity, Amortized Cost, 1 to 5 years	11,761
Held-to-Maturity, Amortized Cost, 5 to 10 years	6,202
Held-to-Maturity, Amortized Cost, More than 10 years	26,166
Amortized Cost	45,479	68,551
Held-to-Maturity Fair Value
Held-to-Maturity, Fair Value, Within 1 year	1,369
Held-to-Maturity, Fair Value, 1 to 5 years	12,370
Held-to-Maturity, Fair Value, 5 to 10 years	6,866
Held-to-Maturity, Fair Value, More than 10 years	26,439
Fair value	47,044	67,312
Corporate bonds
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, Within 1 year	170,007
Available-for-Sale, Amortized Cost, 1 to 5 years	27,624
Available-for-Sale, Amortized Cost, 5 to 10 years	3,500
Available-for-Sale, Amortized Cost, More than 10 years	1,000
Amortized Cost	202,131	200,582
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	170,380
Available-for-Sale, Fair Value, 1 to 5 years	28,171
Available-for-Sale, Fair Value, 5 to 10 years	3,542
Available-for-Sale, Fair Value, More than 10 years	998
Fair Value	203,091	199,163
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
Available-for-Sale, Amortized Cost, 1 to 5 years	1,585
Available-for-Sale, Amortized Cost, More than 10 years	102,713
Amortized Cost	104,298	184,683
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, 1 to 5 years	1,578
Available-for-Sale, Fair Value, More than 10 years	101,791
Fair Value	103,369	$ 182,676
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
Total securities other than mortgage-backed securities
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, Within 1 year	200,819
Available-for-Sale, Amortized Cost, 1 to 5 years	206,829
Available-for-Sale, Amortized Cost, 5 to 10 years	26,085
Available-for-Sale, Amortized Cost, More than 10 years	243,211
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	201,281
Available-for-Sale, Fair Value, 1 to 5 years	210,300
Available-for-Sale, Fair Value, 5 to 10 years	27,308
Available-for-Sale, Fair Value, More than 10 years	251,714
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, Within 1 year	1,350
Held-to-Maturity, Amortized Cost, 1 to 5 years	11,761
Held-to-Maturity, Amortized Cost, 5 to 10 years	6,202
Held-to-Maturity, Amortized Cost, More than 10 years	26,166
Held-to-Maturity Fair Value
Held-to-Maturity, Fair Value, Within 1 year	1,369
Held-to-Maturity, Fair Value, 1 to 5 years	12,370
Held-to-Maturity, Fair Value, 5 to 10 years	6,866
Held-to-Maturity, Fair Value, More than 10 years	26,439
Residential mortgage-backed securities
Available-for-Sale, Amortized Cost
Amortized Cost	1,281,498
Available-for-Sale, Fair Value
Fair Value	1,299,025
Held-to-Maturity, Amortized Cost
Amortized Cost	153,967
Held-to-Maturity Fair Value
Fair value	155,287
Commercial mortgage-backed securities
Available-for-Sale, Amortized Cost
Amortized Cost	282,165
Available-for-Sale, Fair Value
Fair Value	284,953
Held-to-Maturity, Amortized Cost
Amortized Cost	84,087
Held-to-Maturity Fair Value
Fair value	$ 85,573