Regulatory Capital - Well Capitalized Regulatory Requirements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Actual Amount
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	$ 2,989,525	$ 2,896,091
Tier 1 Capital (to Risk-Weighted Assets)	2,989,525	2,896,091
Total Capital (to Risk-Weighted Assets)	3,303,905	3,196,279
Tier 1 Capital (to Average Assets)	$ 2,989,525	$ 2,896,091
Actual Ratio
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	11.90%	12.10%
Tier 1 Capital (to Risk-Weighted Assets)	11.90%	12.10%
Total Capital (to Risk-Weighted Assets)	13.20%	13.30%
Tier 1 Capital (to Average Assets)	11.00%	11.10%
Standardized Approach
Actual Amount
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	$ 2,989,525	$ 2,896,091
Tier 1 Capital (to Risk-Weighted Assets)	2,989,525	2,896,091
Total Capital (to Risk-Weighted Assets)	3,303,905	3,196,279
Tier 1 Capital (to Average Assets)	$ 2,989,525	$ 2,896,091
Actual Ratio
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	11.90%	12.10%
Tier 1 Capital (to Risk-Weighted Assets)	11.90%	12.10%
Total Capital (to Risk-Weighted Assets)	13.20%	13.30%
Tier 1 Capital (to Average Assets)	11.00%	11.10%
Well Capitalized Regulatory Requirements, Amount
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	$ 1,631,899	$ 1,558,174
Tier 1 Capital (to Risk-Weighted Assets)	2,008,491	1,917,752
Total Capital (to Risk-Weighted Assets)	2,510,614	2,397,190
Tier 1 Capital (to Average Assets)	$ 1,356,757	$ 1,299,032
Well Capitalized Regulatory Requirements, Ratio
Common Equity Tier 1 Capital (to Risk-Weighted Assets)	6.50%	6.50%
Tier 1 Capital (to Risk-Weighted Assets)	8.00%	8.00%
Total Capital (to Risk-Weighted Assets)	10.00%	10.00%
Tier 1 Capital (to Average Assets)	5.00%	5.00%