Fair Value Measurements - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets
Loans held-for-sale	$ 460,172	$ 91,202
Derivative assets	298,941	106,102
Liabilities
Derivative liabilities	16,338	9,203
Interest rate contracts
Assets
Derivative assets	279,710	102,401
Liabilities
Derivative liabilities	14,085	5,549
Risk participation agreements
Assets
Derivative assets	163	202
Liabilities
Derivative liabilities	171	354
Forward foreign exchange contracts
Assets
Derivative assets	34	0
Liabilities
Derivative liabilities	970	2,305
Interest rate lock commitments
Assets
Derivative assets	18,592	2,765
Liabilities
Derivative liabilities	0	13
Forward loan sales commitments
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	669	248
Power Equity CDs
Assets
Derivative assets	442	734
Liabilities
Derivative liabilities	443	734
Recurring
Assets
Investment securities available-for-sale	7,446,163	6,720,001
Loans held-for-sale	460,172	91,202
Interest-only strips	9,555	12,813
Derivative assets	301,802	106,642
Forward loan sales commitments, non-derivative		46
Assets held in trust for deferred compensation plans	46,210	43,743
Total assets at fair value	8,263,902	6,974,447
Liabilities
Derivative liabilities	15,840	14,312
Liabilities held in trust for deferred compensation plans	46,210	43,743
Total liabilities at fair value	62,050	58,055
Recurring \| Interest rate contracts
Assets
Derivative assets	279,710	102,893
Liabilities
Derivative liabilities	14,085	6,040
Recurring \| Risk participation agreements
Assets
Derivative assets	163	202
Liabilities
Derivative liabilities	171	354
Recurring \| Forward foreign exchange contracts
Assets
Derivative assets	2,774
Liabilities
Derivative liabilities	210	6,519
Recurring \| Interest rate lock commitments
Assets
Derivative assets	18,596	2,772
Liabilities
Derivative liabilities	4	20
Recurring \| Forward loan sales commitments
Assets
Derivative assets	117	41
Liabilities
Derivative liabilities	786	289
Recurring \| Power Equity CDs
Assets
Derivative assets	442	734
Liabilities
Derivative liabilities	443	734
Recurring \| Swap agreement
Liabilities
Derivative liabilities	141	356
Recurring \| Level 1
Assets
Investment securities available-for-sale	0	0
Loans held-for-sale	0	0
Interest-only strips	0	0
Derivative assets	0	0
Forward loan sales commitments, non-derivative		0
Assets held in trust for deferred compensation plans	46,210	43,743
Total assets at fair value	46,210	43,743
Liabilities
Derivative liabilities	0	0
Liabilities held in trust for deferred compensation plans	46,210	43,743
Total liabilities at fair value	46,210	43,743
Recurring \| Level 1 \| Interest rate contracts
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 1 \| Risk participation agreements
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 1 \| Forward foreign exchange contracts
Assets
Derivative assets	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 1 \| Interest rate lock commitments
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 1 \| Forward loan sales commitments
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 1 \| Power Equity CDs
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 1 \| Swap agreement
Liabilities
Derivative liabilities	0	0
Recurring \| Level 2
Assets
Investment securities available-for-sale	7,445,696	6,719,568
Loans held-for-sale	460,172	91,202
Interest-only strips	0	0
Derivative assets	301,802	106,642
Forward loan sales commitments, non-derivative		46
Assets held in trust for deferred compensation plans	0	0
Total assets at fair value	8,207,670	6,917,458
Liabilities
Derivative liabilities	15,699	13,956
Liabilities held in trust for deferred compensation plans	0	0
Total liabilities at fair value	15,699	13,956
Recurring \| Level 2 \| Interest rate contracts
Assets
Derivative assets	279,710	102,893
Liabilities
Derivative liabilities	14,085	6,040
Recurring \| Level 2 \| Risk participation agreements
Assets
Derivative assets	163	202
Liabilities
Derivative liabilities	171	354
Recurring \| Level 2 \| Forward foreign exchange contracts
Assets
Derivative assets	2,774
Liabilities
Derivative liabilities	210	6,519
Recurring \| Level 2 \| Interest rate lock commitments
Assets
Derivative assets	18,596	2,772
Liabilities
Derivative liabilities	4	20
Recurring \| Level 2 \| Forward loan sales commitments
Assets
Derivative assets	117	41
Liabilities
Derivative liabilities	786	289
Recurring \| Level 2 \| Power Equity CDs
Assets
Derivative assets	442	734
Liabilities
Derivative liabilities	443	734
Recurring \| Level 2 \| Swap agreement
Liabilities
Derivative liabilities	0	0
Recurring \| Level 3
Assets
Investment securities available-for-sale	467	433
Loans held-for-sale	0	0
Interest-only strips	9,555	12,813
Derivative assets	0	0
Forward loan sales commitments, non-derivative		0
Assets held in trust for deferred compensation plans	0	0
Total assets at fair value	10,022	13,246
Liabilities
Derivative liabilities	141	356
Liabilities held in trust for deferred compensation plans	0	0
Total liabilities at fair value	141	356
Recurring \| Level 3 \| Interest rate contracts
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 3 \| Risk participation agreements
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 3 \| Forward foreign exchange contracts
Assets
Derivative assets	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 3 \| Interest rate lock commitments
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 3 \| Forward loan sales commitments
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 3 \| Power Equity CDs
Assets
Derivative assets	0	0
Liabilities
Derivative liabilities	0	0
Recurring \| Level 3 \| Swap agreement
Liabilities
Derivative liabilities	$ 141	$ 356

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