Allowance for Credit Losses - Amortized Cost Basis by Year of Origination and Credit Quality Indicator (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Credit quality
Total loans and leases	$ 13,499,969,000	$ 13,211,650,000
Loss
Credit quality
Total loans and leases	0	0
Residential mortgage
Credit quality
2020	574,473,000
2019	515,543,000
2018	429,473,000
2017	511,347,000
2016	427,035,000
Prior	1,210,429,000
Revolving Loans Amortized Cost Basis	579,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	172,000
Total amortized cost basis	3,669,051,000
Residential mortgage | 740 and greater
Credit quality
2020	457,209,000
2019	410,369,000
2018	319,988,000
2017	390,552,000
2016	341,709,000
Prior	919,509,000
Total amortized cost basis	2,839,336,000
Residential mortgage | 680 - 739
Credit quality
2020	71,240,000
2019	54,497,000
2018	55,179,000
2017	59,237,000
2016	44,911,000
Prior	162,726,000
Total amortized cost basis	447,790,000
Residential mortgage | 620 - 679
Credit quality
2020	12,653,000
2019	11,595,000
2018	9,753,000
2017	8,559,000
2016	8,942,000
Prior	48,554,000
Total amortized cost basis	100,056,000
Residential mortgage | 550 - 619
Credit quality
2020	1,946,000
2019	2,316,000
2018	3,923,000
2017	5,423,000
2016	4,506,000
Prior	13,256,000
Total amortized cost basis	31,370,000
Residential mortgage | Less than 550
Credit quality
2018	534,000
2017	2,153,000
2016	953,000
Prior	3,480,000
Total amortized cost basis	7,120,000
Residential mortgage | No Score
Credit quality
2020	14,827,000
2019	19,364,000
2018	22,222,000
2017	22,616,000
2016	14,954,000
Prior	49,251,000
Total amortized cost basis	143,234,000
Residential mortgage | Other
Credit quality
2020	16,598,000
2019	17,402,000
2018	17,874,000
2017	22,807,000
2016	11,060,000
Prior	13,653,000
Revolving Loans Amortized Cost Basis	579,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	172,000
Total amortized cost basis	100,145,000
Consumer loans
Credit quality
Total loans and leases		226,304,000
Residential and consumer
Credit quality
2020	2,256,314,000
2019	2,210,885,000
2018	1,739,656,000
2017	1,376,649,000
2016	941,535,000
Prior	2,428,263,000
Revolving Loans Amortized Cost Basis	2,506,839,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	39,828,000
Total loans and leases		6,282,731,000
Total amortized cost basis	13,499,969,000
Residential
Credit quality
2020	574,473,000
2019	515,543,000
2018	429,473,000
2017	511,347,000
2016	427,035,000
Prior	1,210,429,000
Revolving Loans Amortized Cost Basis	852,597,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	12,943,000
Total loans and leases		4,662,175,000
Total amortized cost basis	4,533,840,000
Residential | Residential mortgage
Credit quality
Total loans and leases	3,669,051,000	3,768,936,000
Home equity Line | Home equity
Credit quality
Total loans and leases	864,789,000	893,239,000
Consumer
Credit quality
2020	211,421,000
2019	333,637,000
2018	256,657,000
2017	162,196,000
2016	76,723,000
Prior	33,783,000
Revolving Loans Amortized Cost Basis	351,517,000
Total loans and leases		1,620,556,000
Total amortized cost basis	1,425,934,000
Consumer | 740 and greater
Credit quality
2020	102,091,000
2019	127,590,000
2018	106,359,000
2017	61,204,000
2016	28,613,000
Prior	8,443,000
Revolving Loans Amortized Cost Basis	105,905,000
Total amortized cost basis	540,205,000
Consumer | 680 - 739
Credit quality
2020	67,228,000
2019	100,723,000
2018	77,874,000
2017	42,470,000
2016	20,280,000
Prior	7,302,000
Revolving Loans Amortized Cost Basis	77,445,000
Total amortized cost basis	393,322,000
Consumer | 620 - 679
Credit quality
2020	29,733,000
2019	60,484,000
2018	39,213,000
2017	27,825,000
2016	13,701,000
Prior	5,148,000
Revolving Loans Amortized Cost Basis	39,748,000
Total amortized cost basis	215,852,000
Consumer | 550 - 619
Credit quality
2020	5,927,000
2019	24,376,000
2018	21,050,000
2017	18,784,000
2016	9,388,000
Prior	4,134,000
Revolving Loans Amortized Cost Basis	16,484,000
Total amortized cost basis	100,143,000
Consumer | Less than 550
Credit quality
2020	1,558,000
2019	11,200,000
2018	11,967,000
2017	9,579,000
2016	4,671,000
Prior	1,969,000
Revolving Loans Amortized Cost Basis	6,590,000
Total amortized cost basis	47,534,000
Consumer | No Score
Credit quality
2020	3,998,000
2019	120,000
2018	105,000
2017	115,000
2016	23,000
Revolving Loans Amortized Cost Basis	33,654,000
Total amortized cost basis	38,015,000
Consumer | Other
Credit quality
2020	886,000
2019	9,144,000
2018	89,000
2017	2,219,000
2016	47,000
Prior	6,787,000
Revolving Loans Amortized Cost Basis	71,691,000
Total amortized cost basis	90,863,000
Consumer | Consumer loans
Credit quality
Total loans and leases	1,425,934,000	1,620,556,000
Residential Mortgage.
Credit quality
Total loans and leases		3,768,936,000
Home equity line
Credit quality
Revolving Loans Amortized Cost Basis	852,018,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	12,771,000
Total loans and leases		893,239,000
Total amortized cost basis	864,789,000
Home equity line | 740 and greater
Credit quality
Revolving Loans Amortized Cost Basis	617,973,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	5,600,000
Total amortized cost basis	623,573,000
Home equity line | 680 - 739
Credit quality
Revolving Loans Amortized Cost Basis	163,305,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	3,799,000
Total amortized cost basis	167,104,000
Home equity line | 620 - 679
Credit quality
Revolving Loans Amortized Cost Basis	47,448,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	1,295,000
Total amortized cost basis	48,743,000
Home equity line | 550 - 619
Credit quality
Revolving Loans Amortized Cost Basis	13,210,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	1,464,000
Total amortized cost basis	14,674,000
Home equity line | Less than 550
Credit quality
Revolving Loans Amortized Cost Basis	4,750,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	613,000
Total amortized cost basis	5,363,000
Home equity line | No Score
Credit quality
Revolving Loans Amortized Cost Basis	5,332,000
Total amortized cost basis	5,332,000
Commercial and Industrial
Credit quality
2020	1,061,360,000
2019	355,220,000
2018	249,998,000
2017	86,342,000
2016	59,416,000
Prior	103,299,000
Revolving Loans Amortized Cost Basis	1,227,744,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	26,883,000
Total loans and leases	3,170,262,000	2,743,242,000
Total amortized cost basis	3,170,262,000
Commercial and Industrial | Loans graded
Credit quality
Total loans and leases		2,743,242,000
Commercial and Industrial | Pass
Credit quality
2020	999,763,000
2019	330,209,000
2018	202,595,000
2017	76,154,000
2016	52,391,000
Prior	92,446,000
Revolving Loans Amortized Cost Basis	1,033,278,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	26,148,000
Total loans and leases		2,585,908,000
Total amortized cost basis	2,812,984,000
Commercial and Industrial | Special mention
Credit quality
2020	25,677,000
2019	7,786,000
2018	33,794,000
2017	2,354,000
2016	306,000
Prior	791,000
Revolving Loans Amortized Cost Basis	105,339,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	452,000
Total loans and leases		91,365,000
Total amortized cost basis	176,499,000
Commercial and Industrial | Substandard
Credit quality
2020	23,836,000
2019	2,172,000
2018	2,611,000
2017	1,395,000
2016	4,265,000
Prior	9,375,000
Revolving Loans Amortized Cost Basis	37,883,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	283,000
Total loans and leases		65,969,000
Total amortized cost basis	81,820,000
Commercial and Industrial | Doubtful
Credit quality
Prior	195,000
Total amortized cost basis	195,000
Commercial and Industrial | Other
Credit quality
2020	12,084,000
2019	15,053,000
2018	10,998,000
2017	6,439,000
2016	2,454,000
Prior	492,000
Revolving Loans Amortized Cost Basis	51,244,000
Total amortized cost basis	98,764,000
Commercial real estate
Credit quality
2020	291,428,000
2019	730,993,000
2018	589,721,000
2017	493,883,000
2016	347,453,000
Prior	959,342,000
Revolving Loans Amortized Cost Basis	48,263,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	2,000
Total loans and leases	3,461,085,000	3,463,953,000
Total amortized cost basis	3,461,085,000
Commercial real estate | Loans graded
Credit quality
Total loans and leases		3,463,953,000
Commercial real estate | Pass
Credit quality
2020	289,941,000
2019	625,894,000
2018	544,367,000
2017	447,473,000
2016	305,557,000
Prior	880,689,000
Revolving Loans Amortized Cost Basis	36,258,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	2,000
Total loans and leases		3,327,659,000
Total amortized cost basis	3,130,181,000
Commercial real estate | Special mention
Credit quality
2020	1,487,000
2019	94,217,000
2018	30,760,000
2017	39,438,000
2016	31,211,000
Prior	58,911,000
Revolving Loans Amortized Cost Basis	2,999,000
Total loans and leases		106,331,000
Total amortized cost basis	259,023,000
Commercial real estate | Substandard
Credit quality
2019	10,882,000
2018	14,594,000
2017	6,972,000
2016	10,685,000
Prior	19,240,000
Revolving Loans Amortized Cost Basis	9,006,000
Total loans and leases		29,963,000
Total amortized cost basis	71,379,000
Commercial real estate | Other
Credit quality
Prior	502,000
Total amortized cost basis	502,000
Construction
Credit quality
2020	46,159,000
2019	206,400,000
2018	198,246,000
2017	102,268,000
2016	26,374,000
Prior	56,706,000
Revolving Loans Amortized Cost Basis	26,718,000
Total loans and leases	662,871,000	519,241,000
Total amortized cost basis	662,871,000
Construction | Loans graded
Credit quality
Total loans and leases		519,241,000
Construction | Pass
Credit quality
2020	32,810,000
2019	182,503,000
2018	187,257,000
2017	91,415,000
2016	24,053,000
Prior	42,269,000
Revolving Loans Amortized Cost Basis	26,133,000
Total loans and leases		515,993,000
Total amortized cost basis	586,440,000
Construction | Special mention
Credit quality
2019	515,000
2018	1,647,000
2017	4,749,000
Prior	9,172,000
Total loans and leases		127,000
Total amortized cost basis	16,083,000
Construction | Substandard
Credit quality
2018	538,000
2017	1,840,000
2016	525,000
Prior	1,043,000
Total loans and leases		3,121,000
Total amortized cost basis	3,946,000
Construction | Other
Credit quality
2020	13,349,000
2019	23,382,000
2018	8,804,000
2017	4,264,000
2016	1,796,000
Prior	4,222,000
Revolving Loans Amortized Cost Basis	585,000
Total amortized cost basis	56,402,000
Lease financing
Credit quality
2020	71,473,000
2019	69,092,000
2018	15,561,000
2017	20,613,000
2016	4,534,000
Prior	64,704,000
Total loans and leases	245,977,000	202,483,000
Total amortized cost basis	245,977,000
Lease financing | Loans graded
Credit quality
Total loans and leases		202,483,000
Lease financing | Pass
Credit quality
2020	68,770,000
2019	66,472,000
2018	15,044,000
2017	18,063,000
2016	4,110,000
Prior	63,537,000
Total loans and leases		201,461,000
Total amortized cost basis	235,996,000
Lease financing | Special mention
Credit quality
2019	940,000
2018	158,000
2017	1,376,000
2016	424,000
Prior	643,000
Total loans and leases		1,022,000
Total amortized cost basis	3,541,000
Lease financing | Substandard
Credit quality
2020	2,703,000
2019	1,680,000
2018	359,000
2017	1,174,000
Prior	524,000
Total amortized cost basis	6,440,000
Commercial Lending
Credit quality
2020	1,470,420,000
2019	1,361,705,000
2018	1,053,526,000
2017	703,106,000
2016	437,777,000
Prior	1,184,051,000
Revolving Loans Amortized Cost Basis	1,302,725,000
Revolving Loans Converted to Term Loans Amortized Cost Basis	26,885,000
Total amortized cost basis	$ 7,540,195,000
Commercial Lending | Loans graded
Credit quality
Total loans and leases		6,928,919,000
Commercial Lending | Pass
Credit quality
Total loans and leases		6,631,021,000
Commercial Lending | Special mention
Credit quality
Total loans and leases		198,845,000
Commercial Lending | Substandard
Credit quality
Total loans and leases		$ 99,053,000