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Securities - General Disclosures (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2021
USD ($)
security
shares
Mar. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
security
shares
Debt securities available for sale      
Amortized Cost $ 2,789,408   $ 2,211,543
Gross Unrealized Gains 35,676   50,317
Gross Unrealized Losses (28,129)   (673)
Fair Value 2,796,955   2,261,187
Debt securities available for sale, Amortized Cost      
Due in one year or less 100,016    
Due after one year through five years 291,357    
Due after five years through ten years 303,453    
Due after ten years 2,094,582    
Amortized Cost 2,789,408   2,211,543
Debt securities available for sale, Fair Value      
Due in one year or less 100,789    
Due after one year through five years 297,624    
Due after five years through ten years 309,672    
Due after ten years 2,088,870    
Fair Value 2,796,955   2,261,187
Realized gains and losses related to sales and calls of securities AFS      
Gross security gains 25 $ 1,561  
Gross security (losses)   (5)  
Net gains (losses) on sales of debt securities 25 $ 1,556  
Debt securities available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 1,487,290   221,913
Continuous unrealized losses existing for greater than 12 months, gross 5,047   5,096
Total, gross 1,492,337   227,009
Debt securities available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross (28,080)   (621)
Continuous unrealized losses existing greater than 12 months, gross (49)   (52)
Total, gross $ (28,129)   $ (673)
Debt security portfolio | shares 1,108   1,114
Number of securities in unrealized loss position | security 192   23
Securities in unrealized loss position as a percentage of aggregate carrying value of investments 1.88%   0.30%
Collateral      
Realized gains and losses related to sales and calls of securities AFS      
Carrying amount of investment securities pledged as collateral $ 562,600   $ 628,000
U.S. Treasury securities      
Debt securities available for sale      
Amortized Cost 23,514   27,481
Gross Unrealized Gains 243   356
Fair Value 23,757   27,837
Debt securities available for sale, Amortized Cost      
Amortized Cost 23,514   27,481
Debt securities available for sale, Fair Value      
Fair Value 23,757   27,837
Obligations of U.S. government corporations and agencies      
Debt securities available for sale      
Amortized Cost 64,903   67,406
Gross Unrealized Gains 1,720   2,162
Gross Unrealized Losses (45)   (49)
Fair Value 66,578   69,519
Debt securities available for sale, Amortized Cost      
Amortized Cost 64,903   67,406
Debt securities available for sale, Fair Value      
Fair Value 66,578   69,519
Debt securities available for sale, Fair Value      
Continuous unrealized losses existing for greater than 12 months, gross 4,695   4,957
Total, gross 4,695   4,957
Debt securities available for sale, Unrealized Losses      
Continuous unrealized losses existing greater than 12 months, gross (45)   (49)
Total, gross (45)   (49)
Obligations of states and political subdivisions      
Debt securities available for sale      
Amortized Cost 280,150   292,940
Gross Unrealized Gains 8,854   11,779
Gross Unrealized Losses (1,542)   (8)
Fair Value 287,462   304,711
Debt securities available for sale, Amortized Cost      
Amortized Cost 280,150   292,940
Debt securities available for sale, Fair Value      
Fair Value 287,462   304,711
Debt securities available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 58,555   762
Total, gross 58,555   762
Debt securities available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross (1,542)   (8)
Total, gross (1,542)   (8)
Commercial mortgage-backed securities      
Debt securities available for sale      
Amortized Cost 499,996   408,716
Gross Unrealized Gains 5,308   10,212
Gross Unrealized Losses (6,786)   (312)
Fair Value 498,518   418,616
Debt securities available for sale, Amortized Cost      
Amortized Cost 499,996   408,716
Debt securities available for sale, Fair Value      
Fair Value 498,518   418,616
Debt securities available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 314,109   129,655
Total, gross 314,109   129,655
Debt securities available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross (6,786)   (312)
Total, gross (6,786)   (312)
Residential mortgage-backed securities      
Debt securities available for sale      
Amortized Cost 1,786,062   1,344,047
Gross Unrealized Gains 18,587   24,571
Gross Unrealized Losses (18,329)   (303)
Fair Value 1,786,320   1,368,315
Debt securities available for sale, Amortized Cost      
Amortized Cost 1,786,062   1,344,047
Debt securities available for sale, Fair Value      
Fair Value 1,786,320   1,368,315
Debt securities available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 1,019,462   89,997
Continuous unrealized losses existing for greater than 12 months, gross 352   139
Total, gross 1,019,814   90,136
Debt securities available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross (18,325)   (300)
Continuous unrealized losses existing greater than 12 months, gross (4)   (3)
Total, gross (18,329)   (303)
Corporate debt securities      
Debt securities available for sale      
Amortized Cost 134,783   70,953
Gross Unrealized Gains 964   1,237
Gross Unrealized Losses (1,427)   (1)
Fair Value 134,320   72,189
Debt securities available for sale, Amortized Cost      
Amortized Cost 134,783   70,953
Debt securities available for sale, Fair Value      
Fair Value 134,320   72,189
Debt securities available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 95,164   1,499
Total, gross 95,164   1,499
Debt securities available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross (1,427)   (1)
Total, gross $ (1,427)   $ (1)