Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 3,427,741	$ 3,122,950
Net unrealized gains (losses) arising during the period	$ 10,913	$ 1,990	16,975	17,099
Amounts reclassified from AOCI	(1,576)	(1,644)	(3,010)	(3,870)
Other comprehensive income	9,337	346	13,965	13,229
Ending balance	3,670,261	3,296,910	3,670,261	3,296,910
Available-for-Sale Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(25,151)	6,772	(28,772)	(6,144)
Net unrealized gains (losses) arising during the period	7,777	6,628	12,832	21,770
Amounts reclassified from AOCI	(1,576)	(1,644)	(3,010)	(3,870)
Other comprehensive income	6,201	4,984	9,822	17,900
Ending balance	(18,950)	11,756	(18,950)	11,756
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(18,367)	(8,830)	(19,374)	(8,797)
Net unrealized gains (losses) arising during the period	3,136	(4,638)	4,143	(4,671)
Amounts reclassified from AOCI	0	0	0	0
Other comprehensive income	3,136	(4,638)	4,143	(4,671)
Ending balance	(15,231)	(13,468)	(15,231)	(13,468)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(43,518)	(2,058)	(48,146)	(14,941)
Other comprehensive income			13,965	13,229
Ending balance	$ (34,181)	$ (1,712)	$ (34,181)	$ (1,712)