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, 2018	Dec. 31, 2017
Available-for-Sale, Amortized Cost
Amortized Cost	$ 2,660,263	$ 2,623,728
Available-for-Sale, Fair Value
Fair Value	2,628,467	2,615,850
Held-to-Maturity, Amortized Cost
Amortized Cost	274,407	321,094
Held-to-Maturity Fair Value
Fair value	268,803	321,276
U.S. Treasuries
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, 1 to 5 years	122,563
Available-for-Sale, Amortized Cost, 5 to 10 years	28,149
Amortized Cost	150,712	122,025
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, 1 to 5 years	120,391
Available-for-Sale, Fair Value, 5 to 10 years	28,916
Fair Value	149,307	121,113
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
Amortized Cost	0
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
Fair value	0
US Government agencies
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, 1 to 5 years	20,794
Available-for-Sale, Amortized Cost, More than 10 years	4,699
Amortized Cost	25,493
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, 1 to 5 years	20,538
Available-for-Sale, Fair Value, More than 10 years	5,015
Fair Value	25,553
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	500
Available-for-Sale, Amortized Cost, 1 to 5 years	43,526
Available-for-Sale, Amortized Cost, 5 to 10 years	41,841
Available-for-Sale, Amortized Cost, More than 10 years	148,883
Amortized Cost	234,750	195,663
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	504
Available-for-Sale, Fair Value, 1 to 5 years	43,045
Available-for-Sale, Fair Value, 5 to 10 years	41,771
Available-for-Sale, Fair Value, More than 10 years	148,621
Fair Value	233,941	197,286
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, Within 1 year	5,830
Held-to-Maturity, Amortized Cost, 1 to 5 years	10,968
Held-to-Maturity, Amortized Cost, 5 to 10 years	8,354
Held-to-Maturity, Amortized Cost, More than 10 years	43,399
Amortized Cost	68,551	71,959
Held-to-Maturity Fair Value
Held-to-Maturity, Fair Value, Within 1 year	5,859
Held-to-Maturity, Fair Value, 1 to 5 years	11,317
Held-to-Maturity, Fair Value, 5 to 10 years	8,928
Held-to-Maturity, Fair Value, More than 10 years	41,208
Fair value	67,312	73,355
Corporate bonds
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, 1 to 5 years	198,082
Available-for-Sale, Amortized Cost, 5 to 10 years	1,500
Available-for-Sale, Amortized Cost, More than 10 years	1,000
Amortized Cost	200,582	305,265
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, 1 to 5 years	196,654
Available-for-Sale, Fair Value, 5 to 10 years	1,514
Available-for-Sale, Fair Value, More than 10 years	995
Fair Value	199,163	306,353
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	0
Amortized Cost	0
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	0
Fair value	0
Asset-backed securities
Available-for-Sale, Amortized Cost
Available-for-Sale, Amortized Cost, 5 to 10 years	2,305
Available-for-Sale, Amortized Cost, More than 10 years	182,378
Amortized Cost	184,683	236,533
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, 5 to 10 years	2,300
Available-for-Sale, Fair Value, More than 10 years	180,376
Fair Value	182,676	237,458
Held-to-Maturity, Amortized Cost
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, 5 to 10 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	500
Available-for-Sale, Amortized Cost, 1 to 5 years	384,965
Available-for-Sale, Amortized Cost, 5 to 10 years	73,795
Available-for-Sale, Amortized Cost, More than 10 years	336,960
Available-for-Sale, Fair Value
Available-for-Sale, Fair Value, Within 1 year	504
Available-for-Sale, Fair Value, 1 to 5 years	380,628
Available-for-Sale, Fair Value, 5 to 10 years	74,501
Available-for-Sale, Fair Value, More than 10 years	335,007
Held-to-Maturity, Amortized Cost
Held-to-Maturity, Amortized Cost, Within 1 year	5,830
Held-to-Maturity, Amortized Cost, 1 to 5 years	10,968
Held-to-Maturity, Amortized Cost, 5 to 10 years	8,354
Held-to-Maturity, Amortized Cost, More than 10 years	43,399
Held-to-Maturity Fair Value
Held-to-Maturity, Fair Value, Within 1 year	5,859
Held-to-Maturity, Fair Value, 1 to 5 years	11,317
Held-to-Maturity, Fair Value, 5 to 10 years	8,928
Held-to-Maturity, Fair Value, More than 10 years	41,208
Residential mortgage-backed securities
Available-for-Sale, Amortized Cost
Amortized Cost	1,464,380	1,317,371
Available-for-Sale, Fair Value
Fair Value	1,445,910	1,311,733
Held-to-Maturity, Amortized Cost
Amortized Cost	176,488	208,805
Held-to-Maturity Fair Value
Fair value	172,046	207,102
Commercial mortgage-backed securities
Available-for-Sale, Amortized Cost
Amortized Cost	399,663	420,685
Available-for-Sale, Fair Value
Fair Value	391,917	415,478
Held-to-Maturity, Amortized Cost
Amortized Cost	29,368	40,330
Held-to-Maturity Fair Value
Fair value	$ 29,445	$ 40,819