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Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Amortized Cost    
Due in one year or less $ 2,062 $ 1,349
Due after one through five years 4,781 5,545
Due after five through ten years 92,598 70,777
Due after ten years 1,218,472 1,097,451
Total debt securities with a single maturity date 1,317,913 1,175,122
Total investment securities 2,343,573 1,807,389
Fair Value    
Due in one year or less 2,071 1,353
Due after one through five years 4,960 5,764
Due after five through ten years 96,882 75,223
Due after ten years 1,278,792 1,182,110
Total debt securities with a single maturity date 1,382,705 1,264,450
Total investment securities 2,405,568 1,919,119
Amortized Cost    
Due in one year or less 9,056 9,712
Due after one through five years 23,116 22,241
Due after five through ten years 151,103 115,408
Due after ten years 503,454 505,153
Total debt securities with a single maturity date 686,729 652,514
Total investment securities 1,295,534 1,227,668
Fair Value    
Due in one year or less 9,068 9,755
Due after one through five years 24,084 23,190
Due after five through ten years 154,819 121,333
Due after ten years 516,971 533,079
Total debt securities with a single maturity date 704,942 687,357
Investment securities held to maturity 1,318,780 1,280,293
U.S. Government-sponsored mortgage-backed securities    
Amortized Cost    
Without single maturity date 1,025,660 632,267
Total investment securities 1,025,660 632,267
Fair Value    
Without single maturity date 1,022,863 654,669
Total investment securities 1,022,863 654,669
Amortized Cost    
Without single maturity date 608,805 575,154
Total investment securities 608,805 575,154
Fair Value    
Without single maturity date 613,838 592,936
Investment securities held to maturity $ 613,838 $ 592,936