Summary of Significant Accounting Policies Error Corrections to Cash Flows (Details) - USD ($)	3 Months Ended		6 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash used in investing activities							$ (740,717,000)	$ (1,135,331,000)	$ (1,037,616,000)
Net (decrease) increase in cash and restricted cash							43,591,000	49,570,000	21,106,000
Cash, beginning of period	$ 108,291,000		$ 108,291,000		$ 108,291,000		108,291,000
Cash, end of period							103,436,000	108,291,000
Cash and Cash Equivalents at Beginning of Year	402,096,000	$ 352,526,000	402,096,000	$ 352,526,000	402,096,000	$ 352,526,000	402,096,000	352,526,000	331,420,000
Cash and Cash Equivalents at End of Year							445,687,000	402,096,000	352,526,000
Immaterial Misstatement Of Cash And Restricted Cash In Cash Flows
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease (increase) in other interest-earning assets	59,034,000	(76,087,000)	4,312,000	(57,819,000)	(39,974,000)	(341,385,000)
Net cash used in investing activities	8,989,000	(296,821,000)	(209,407,000)	(642,214,000)	(469,515,000)	(1,167,001,000)
Net (decrease) increase in cash and restricted cash	(35,866,000)	(41,871,000)	5,999,000	19,946,000	(24,214,000)	(5,801,000)
Cash and Cash Equivalents at Beginning of Year	159,304,000	144,812,000	159,304,000	144,812,000	159,304,000	144,812,000	159,304,000	144,812,000
Cash and Cash Equivalents at End of Year	123,438,000	102,941,000	165,303,000	164,758,000	135,090,000	139,011,000		159,304,000	144,812,000
As Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease (increase) in other interest-earning assets	86,760,000	(59,135,000)	(3,480,000)	(71,845,000)	(49,225,000)	(376,696,000)
Net cash used in investing activities	36,715,000	(279,869,000)	(217,199,000)	(656,240,000)	(478,766,000)	(1,202,312,000)
Net (decrease) increase in cash and restricted cash	(8,140,000)	(24,919,000)
Net (decrease) increase in cash and restricted cash			(1,793,000)	5,920,000	(33,465,000)	(41,112,000)
Cash, beginning of period	108,291,000	118,763,000	108,291,000	118,763,000	108,291,000	118,763,000	108,291,000	118,763,000
Cash, end of period	100,151,000	93,844,000						108,291,000	118,763,000
Cash and Cash Equivalents at Beginning of Year	$ 246,726,000	$ 236,887,000	246,726,000	236,887,000	246,726,000	236,887,000	$ 246,726,000	236,887,000
Cash and Cash Equivalents at End of Year			$ 244,933,000	$ 242,807,000	$ 213,261,000	$ 195,775,000		$ 246,726,000	$ 236,887,000