Quarterly Financial Data - Unaudited (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating results:
Interest income	$ 56,334	$ 60,906	$ 66,787	$ 62,328	$ 68,829	$ 63,484	$ 63,373	$ 63,746	$ 246,355	$ 259,432	$ 242,070
Interest expense	(20,537)	(21,597)	(22,444)	(23,950)	(25,039)	(23,875)	(23,008)	(23,961)	(88,528)	(95,883)	(87,463)
Net Interest Income	35,797	39,309	44,343	38,378	43,790	39,609	40,365	39,785	157,827	163,549	154,607
Non-interest income	9,763	33,712	10,888	4,454	19,593	(3,412)	14,104	(5,031)	58,817	25,254	37,790
Operating expenses	(17,824)	(20,355)	(20,155)	(30,452)	(22,638)	(24,497)	(25,218)	(25,063)	(88,786)	(97,416)	(90,123)
Net Income	$ 25,355	$ 52,553	$ 41,281	$ 12,063	$ 41,059	$ 19,164	$ 27,064	$ 14,801	$ 131,252	$ 102,088	$ 100,569
Per share data:
Basic earnings per common share (in dollars per share)	$ 0.32	$ 0.67	$ 0.52	$ 0.15	$ 0.49	$ 0.22	$ 0.31	$ 0.17	$ 1.66	$ 1.20	$ 1.18
Diluted earnings per common share (in dollars per share)	0.31	0.58	0.48	0.15	0.46	0.22	0.31	0.16	1.54	1.18	1.15
Regular dividends declared per common share (in dollars per share)	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 1.12	$ 1.12	$ 1.12
Yield maintenance fees received	$ 1,000	$ 1,000	$ 5,000		$ 2,000		$ 2,000		$ 2,000
Reversal of provision for loan losses	0	859	$ 6,532	$ (289)					7,102	$ 355	$ (961)
Loans and Leases Receivable Disclosure [Line Items]
Costs incurred and expensed				$ 10,401					10,401	$ 0	$ 0
Commercial Loans At Lower Of Cost Or Market
Per share data:
Yield maintenance fees received									$ 16,000
Loans and Leases Receivable Disclosure [Line Items]
Realized gains, net	$ 1,000	$ 5,000