Shareholders' Equity (Capital) (Details 1) $ / shares in Units, $ in Thousands	2 Months Ended	12 Months Ended			198 Months Ended
	Feb. 14, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) Y $ / shares shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2014 USD ($)
Minimum required capital amounts and ratios for well capitalized institutions and the actual capital amounts [Line Items]
Shareholders' Equity		$ 1,231,868	$ 1,161,537	$ 1,116,260	$ 1,231,868	$ 1,055,086
Common Equity Tier One Capital		1,238,063	1,168,228		1,238,063
Tier 1 Capital		1,238,063	1,168,228		1,238,063
Total Capital		$ 1,352,231	$ 1,278,528		$ 1,352,231
Well Capitalized Minimum Common Equity Tier One Ratio (as a percent)		6.50%	6.50%		6.50%
Common Equity Tier One Ratio		13.24%	13.24%		13.24%
Well Capitalized Minimum Tier 1 Capital Ratio (as a percent)		8.00%	8.00%		8.00%
Tier 1 Capital Ratio (as a percent)		13.24%	13.24%		13.24%
Well Capitalized Minimum Total Capital Ratio (as a percent)		10.00%	10.00%		10.00%
Total Capital Ratio (as a percent)		14.46%	14.49%		14.46%
Well Capitalized Minimum Tier 1 Leverage Ratio (as a percent)		5.00%	5.00%		5.00%
Tier 1 Leverage Ratio (as a percent)		7.26%	7.21%		7.26%
Dividends
Number of prior calendar years considered for payment of dividends in excess of the sum of net income | Y		2
Common Stock Repurchase Program
Number of shares of common stock repurchased | shares		549,199			54,200,000
Amount returned to shareholders on stock repurchase		$ 47,076	$ 61,807	$ 52,981	$ 2,000,000
Average cost of shares repurchased (in dollars per share) | $ / shares		$ 81.91			$ 38.29
Treasury Stock Value Acquired Cost Method Share Repurchase Program		$ 45,000
Bank
Minimum required capital amounts and ratios for well capitalized institutions and the actual capital amounts [Line Items]
Shareholders' Equity		1,161,037	1,094,461		$ 1,161,037
Common Equity Tier One Capital		1,178,804	1,111,525		1,178,804
Tier 1 Capital		1,178,804	1,111,525		1,178,804
Total Capital		$ 1,292,972	$ 1,221,683		$ 1,292,972
Common Equity Tier One Ratio		12.62%	12.61%		12.62%
Tier 1 Capital Ratio (as a percent)		12.62%	12.61%		12.62%
Total Capital Ratio (as a percent)		13.85%	13.86%		13.85%
Tier 1 Leverage Ratio (as a percent)		6.92%	6.86%		6.92%
Subsequent Event
Common Stock Repurchase Program
Number of shares of common stock repurchased | shares	80,000
Amount returned to shareholders on stock repurchase	$ 6,700
Average cost of shares repurchased (in dollars per share) | $ / shares	$ 84.27